Color separation screen printing work splits a design into one film per ink color, each printed as solid black on its own layer or channel; you then add an underbase for dark shirts, set trap or choke where colors meet, and output the films at true size, with matching registration marks, in print order.

Core rule: one ink, one plate, one screen. Every separation decision, from the underbase to the halftones, comes back to what each screen will put on the shirt and in what order.

Part of the screen printing guides in the DAX Learning Hub. Choosing between spot color, simulated process and four color process is covered in Simulated Process vs Spot Color. File formats, resolution and fonts are covered in Artwork File Prep for Apparel. This page covers the separation work itself.

Published by DAX (Decorated Apparel Expo) · Last updated: October 2026

Color Separation Screen Printing Types: What Each Film Carries

Separations come in a few families. The table below describes what the films look like for each one, not which method to pick. For the choice itself, use the method comparison guide.

How to Color Separate a Design Step by Step

The steps below work for most spot color and index jobs in any separation program. Simulated process follows the same order with more tonal plates.

1. Confirm the order: garment color, print size, placement and the number of screens your press can hold.
2. Open the art at final print size and clean it up: remove stray pixels, close gaps and decide which areas are really the shirt color.
3. Pick the inks. List each color the design needs and merge near duplicates so every plate earns its screen.
4. Split the art into one plate per ink, with each plate shown as solid black where that ink prints.
5. On a dark garment, build a white underbase from the areas that need it and choke it slightly inside the top colors.
6. Set traps where colors touch, usually spreading the lighter ink a little under the darker one.
7. Add halftones to any plate with soft edges or shading, matched to your mesh and press.
8. Set the print order and decide whether a highlight white prints last.
9. Output each plate as a film positive at true size with the same registration marks, a label and the right orientation for your exposure method.
10. Check the stack of films against the proof before burning screens.

Underbase, Trap and Choke

On dark shirts, most inks are not opaque enough to hold their color on their own, so a white underbase is printed first, flashed, and the colors print on top. If the underbase is the exact shape of the top colors, any small shift in registration leaves a thin white halo. Choking the underbase, pulling its edges slightly inward, hides it under the colors.

Trap is the same idea between two top colors. Spreading the lighter ink a little under its darker neighbor means a hairline gap never shows the shirt. Guides commonly describe choke and trap in fractions of a millimeter or fractions of a point, more on loose screens or manual presses where registration moves. Treat any published number as a starting point and set yours by test prints on your own press.

A highlight white is a second white printed last, on top, for the brightest areas. It is a separate screen from the underbase. Underbase strategy, flash timing and opacity are covered in depth in Underbase Screen Printing.

Halftones Inside Separations

Spot color art with gradients, shadows or glows needs halftones on those plates, or the soft areas print as flat blocks. Simulated process and four color process are built almost entirely from halftone plates. Index seps avoid halftone angles by using the dither, though the pixel size still has to suit the mesh.

Line count, dot shape, angle and dot gain are mesh and press decisions, so they belong in your tested recipe rather than in the art file. The halftone screen printing guide covers those settings.

Print Order

A common convention for spot color work is to print lighter inks first and darker inks last, because a dark ink covers a light one cleanly where they meet while light inks pick up contamination from dark ones. It is a convention, not a law. Some simulated process and index separations run darker to lighter so the brightest tones and highlights stay clean. Write the order on the films and the job ticket so the press crew does not have to guess.

Checking Seps Before Burning Screens

Most separation mistakes are visible on film before a screen is exposed. Lay the films over each other on a light table or window and work through a short list:

1. Size matches the order and the art fits the film with its marks and labels.
2. Every film carries the same registration marks in the same place.
3. Each film is labeled with the job, ink and plate number.
4. Orientation matches how you expose, right reading or mirrored.
5. The underbase sits just inside the top colors, and lighter inks tuck under darker ones.
6. Black areas are dense and solid, with no gray or broken dots.
7. The plate count and order fit the press.

Film output, density and printer setup are covered in Screen Printing Film Positives. A few minutes with the stack saves a reclaimed screen and a lost press slot.

Frequently Asked Questions About Color Separation (FAQ)

How do you color separate a design for screen printing?

Split the art into one plate per ink, shown as solid black, add an underbase for dark shirts, set choke and trap, add halftones where needed, then output true size films with matching registration marks in print order.

How many screens does a design need?

One per ink color, plus an underbase on dark garments and sometimes a highlight white. Merging similar colors is the usual way to bring the count down.

What is trapping in screen printing?

Trapping is a small overlap where two colors meet, usually made by spreading the lighter color under the darker one, so a slight registration shift does not leave a gap showing the shirt.

Do I need an underbase on dark shirts?

Usually, for bright colors. Most inks are not fully opaque on dark fabric, so a choked white underbase printed and flashed first keeps the top colors bright. See the underbase guide.

Can vector art be separated automatically?

Flat vector art with defined colors often separates cleanly with automatic tools, since each color already sits on its own shapes. Gradients, effects and photos still need judgment, and every automatic result should be checked on film before exposure.

Supplier note: figures here are hedged starting points that vary by garment, ink, paint, film and equipment; confirm every setting against the supplier or manufacturer spec sheet, the product label and the garment care label. Choke, trap, halftone and resolution numbers in published guides are starting points; set yours from test prints on your own mesh, press and ink.

Browse more guides in the DAX Learning Hub, and check cities and dates on the DAX show schedule when you are ready to see screen printing equipment, inks and art services in person.