How to separate colors in Photoshop for screen printing

Step 1 — set the document up before you separate anything

Resolution first: work at final print size at 300 dpi. Not the size the file arrived at, and not 72 dpi with a plan to scale later — every choke, spread and trap value you set from here is measured in pixels, so they only mean something if the pixel size is fixed and correct. Some shops work at 600 dpi for finer control on lettering; the trade is file size. Then clean the art. Remove the background if the design arrived on one. Kill JPEG noise around edges, because compression fringe becomes speckle on a plate and a fuzzy edge on film. If the design is going on a dark garment, put a solid layer of the garment colour underneath everything now — you will need it to judge the separation later, and adding it afterwards means re-checking work you thought was done. Finally, note down the ink count you are actually willing to burn. Separating to nine inks and discovering the press has six stations is a wasted afternoon.

Step 2 — turn each ink into a spot channel with Color Range

This is the core move. Choose Select > Color Range, click a pixel of the first ink in the image, and adjust Fuzziness until the selection covers every appearance of that ink and nothing else — Fuzziness is your tolerance. Leave Localized Color Clusters OFF, which is the opposite of what a lot of tutorials imply. It does not gather scattered same-coloured areas together; it does the reverse, restricting the selection to pixels near where you clicked, with a Range slider controlling how near. It exists for when you want only one of several identically-coloured objects. For a screen separation you almost never do: a plate is one ink everywhere it appears, so the eleven letters of a cream wordmark plus two cream snowcaps have to land in one selection. Tick that box and you fragment the wordmark across plates, which is the exact failure a separation is supposed to prevent. Then, with the selection still active, open the Channels panel menu and choose New Spot Channel. Photoshop creates a channel and fills your selection with the ink at full strength. Click the colour swatch in the dialog and set it to the actual ink you will mix, and name the channel after that ink rather than leaving it "Spot Color 1" — future you, at the press, will care. Ignore the temptation to fiddle with Solidity: it changes the on-screen preview of ink density only and has no effect whatsoever on the printed separation. Repeat per ink. The thing to hold in your head is that a spot channel stores ink coverage, so black in the channel means 100% ink and white means bare garment — the opposite of how an alpha mask reads, which trips up nearly everybody the first time.

Step 3 — fix density with Curves, one channel at a time

A raw Color Range selection is binary and blunt: it either grabbed a pixel or it did not, and where two inks blend you will get hard edges or gaps. Select a single channel in the Channels panel and then apply Image > Adjustments > Curves, and you are adjusting only that plate's ink density. Pull a channel back that is fighting its neighbour and printing too heavy; push one that came out too thin to hold on press. Work with only the plate you are adjusting plus the garment layer visible, then turn everything back on and look at the composite. Two failures to hunt for at this stage: a gap between two adjacent inks, which will show garment through it after any misregistration, and a plate that is technically present but so light it will not survive the emulsion. Both are cheaper to fix here than after burning.

Step 4 — build the underbase and choke it

The underbase is the combined coverage of every colour plate, pulled inward. Load your first colour channel as a selection by Ctrl-clicking (Cmd-clicking on Mac) its thumbnail in the Channels panel, then Shift-Ctrl-click each remaining colour channel to add it. Now invert it — Select > Inverse — and this step is not optional. Remember from Step 2 that a spot channel stores ink coverage, so black means 100% ink; loading a channel as a selection selects its light areas, which is everywhere the ink will NOT land. Skip the inverse and you will choke and build a base for the empty space around your design and wonder why the print has a white rectangle in it. After inverting, you have a selection of everywhere any ink will land. Next, choke it: Select > Modify > Contract, and enter a small number of pixels. Get the units right, because they are widely misquoted: a point is 1/72 inch, so at 300 dpi one pixel is 0.24 pt — a quarter point — and a half-point choke is about 2 pixels at 300 dpi or 4 at 600. The usable range is roughly 1 to 3 pixels at 300 dpi. Then make a new spot channel from that contracted selection, name it Underbase or White Base, and put it first in the print order. An alternative many printers use directly in the channel — Scott Fresener's method — is to select the solid areas of the underbase channel, then Edit > Stroke with white, a width of 4 to 6 pixels, and Location set to Center: because the stroke straddles the edge it eats in by about half its width, so 2 to 3 pixels at 300 dpi, and firms up a soft edge at the same time. Note that this is a heavier choke than a one-pixel contract, not the same move in different clothing — it is two to three times as much, landing at the top of the range above. Pick one method and one number rather than mixing them. Note also that Select > Modify is greyed out unless a selection is currently active; if you deselected, reload the channels. And if the artwork has gradients or halftoned areas, do not threshold the base to solid white — let it carry the tone, or a 40% shadow gets a slab of white under it and lights up from below.

Step 5 — highlight white is a separate channel

The white under the print and the white on top of it are two different plates doing two different jobs, and merging them is the most common reason dark-garment prints come out flat. For the highlight white, take the brightest values only: duplicate a channel or the composite luminosity, then use Levels or Curves to crush everything except the top of the range, so what remains is the specular hits — the glint in an eye, the top edge of chrome lettering, the crest of a splash. Make that its own spot channel and put it last in the print order, over everything. Set the threshold by eye against the garment colour: too low and your highlight white creeps into midtones and chalks the whole print; too high and the design loses its snap. If the artwork has no true whites in it, skip the plate entirely.

Step 6 — halftone the plates, if there is no RIP in the chain

Skip this step if you print through a RIP. A RIP wants continuous-tone greyscale plates and will screen them itself with curves matched to its output device; hand it film you have already screened and it re-screens your dots, which moirés against your own artwork. If you print film straight to an inkjet or laser through a plain driver, the screening has to happen in Photoshop. The route is per plate: copy the channel into its own document, Image > Mode > Grayscale, then Image > Mode > Bitmap, and choose Halftone Screen as the method. You get three controls — Frequency in lines per inch, Angle, and Shape (round, diamond, ellipse, line, square or cross). Set frequency from your mesh, not your ambition: the rule of thumb in circulation puts mesh count around four to five-and-a-half times your lpi, so 45 lpi sits comfortably on 205 mesh and 55 lpi pairs with 305, and a finer screen than your mesh can hold simply falls apart. Give each plate a different angle so the grids do not beat against each other, and record which angle went on which plate. Ellipse is a common choice because round dots all touch their neighbours at once around 50% and produce a visible tonal jump.

Step 7 — dot gain, which you earn rather than guess

Ink spreads on fabric, so a 50% dot on film lands well past 50% on the shirt, and it is worst in the midtones where the image has most to lose. The correction is an inverse curve applied to each plate before screening: pull the midtones down so the dot that ends up on the garment is the one you meant. The amount is not a number anyone can give you — it depends on your mesh, emulsion thickness, ink, squeegee, pressure and substrate. Print a step wedge once on your own setup, read where the values actually land, and derive the curve from that. After that it is the same curve for every job on that press, which makes it a one-off afternoon that pays for itself indefinitely. Skipping this is the single most common cause of muddy process prints.

Step 8 — registration marks, print order, and saving

Add registration marks on a layer or channel that appears identically on every plate, outside the image area, so the films can be lined up on the screens. Write the print order down: underbase first, then colours (shops commonly run darker to lighter, but your inks and your flash decide), then highlight white last. Save as PSD — it preserves spot channels with their ink colours, which is what makes the file behave like a separation rather than a stack of pictures. Image > Mode > Multichannel converts the document to nothing but channels if you want the plates with no composite at all; be aware it flattens layers, so save your working file first. To output the plates individually, solo one channel at a time and export, or duplicate each channel into its own greyscale document. Whatever you do, keep the layered working file — the reprint with one colour swapped is coming.

Simulated process by hand: what you are signing up for

Everything above is a spot separation, and on flat art it is genuinely quick once you have done it twice. Simulated process is a different order of work. You are not selecting inks any more, you are constructing a tonal channel for each ink from a photograph — deciding where a red channel hands off to a magenta, building channels from blends of the composite, curving each one against the others, generating a tonal base under all of it, timing the highlight white, and then screening every plate. It takes hours, it depends on judgement about your specific inks and mesh, and it is the reason separation services charge $60 to $85 for a photorealistic design — a promotional flat $65 per set at Scott Fresener's own 24 Hour Seps, $85 and up at ContractPrinter's top tier — and why printers with the skill are in demand. If you are teaching yourself, do it on a job that is not due tomorrow, and print a test before you promise anything.

The one-button version, and where it stops

Every step above can be done for you, and it is worth knowing exactly where the automation stops being trustworthy. Spot and index separation are genuinely a click: perceptual clustering groups an ink onto one plate wherever it appears — including all eleven letters of a wordmark, which is the case hand-selection most often fudges — and it runs entirely in your browser for free, with no account and no upload. The multichannel PSD is free too, and it is the file this whole page has been building by hand: one real Photoshop spot channel per ink, each carrying its ink colour and name, at the physical DPI implied by your print width, with the garment base left out. Open it and you are already at the end of Step 8, with the plates you would otherwise have spent the afternoon selecting. The print-ready render is the paid part, and it does the rest of this page: the choked underbase from combined coverage, the separate highlight white, per-ink choke and spread, trapping, the dot-gain curve, halftone screening at a generated non-moiré angle set, registration marks, and film at 600 or 1200 dpi. Where it stops: simulated process. That ships with per-channel curve and density override because automatic sim-process is unreliable in every tool that sells it, ours included — so the auto pass is a starting point and the sliders are the method, which is the same conclusion you reach doing it by hand.