Color separation is the step that splits a piece of artwork into one layer per ink, because a printing press lays down one ink at a time from its own screen or plate. Each layer becomes a film or plate that says where that ink goes and how much. The four common ways to split artwork are spot color, CMYK process, index and simulated process.
This guide explains what a separation actually is, how each of the four methods works, and how to choose between spot color and process for a given job.
Why printers separate artwork
Your screen shows a design as millions of mixed colors. A press cannot do that. In screen printing, each ink is pushed through its own mesh screen, one pass after another. In offset printing, each ink has its own plate. A four-color shirt print therefore needs four screens, and each screen needs its own film positive to expose it.
Separation is the translation between the two worlds. It decides how many inks the job uses, which ink covers which area, and how partial tones are faked with dots. Get it right and the colors land where they should. Get it wrong and you pay for screens you did not need, or you see gaps, halos and muddy colors on press.
That is also why separation drives cost. Every extra ink is another screen to coat, expose, register and clean. On most screen-print quotes the color count is one of the biggest numbers.
What a separation looks like
A separated file is a set of grayscale layers, one per ink. In each layer, black means full ink, white means no ink, and anything in between is a partial amount. Because a screen can only print ink or no ink, partial amounts are converted into halftone dots: big dots for dark areas, small dots for light ones. Your eye blends them at normal viewing distance.
Each layer is output as a film positive, solid black where ink goes and clear elsewhere, with matching registration marks so the printer can line up every screen in the same place. On dark garments, there is often an extra layer printed first: a white underbase that makes the colors on top look bright.
Spot color separation
Spot color is the simplest method. Each distinct color in the design gets its own premixed ink and its own screen, so a red, black and yellow logo is three screens. Spot colors print as solid areas, though a spot screen can also carry halftone dots for a tint or a soft fade.
Spot inks are mixed to a target before printing, and designers often specify them with a color system code. Pantone® is the best-known system, and its codes follow fixed formats, such as a code like 19-4052 TCX in its textile range. Adobe's guide to spot vs process color sums up the trade-off well: spot color suits a graphic approach with a limited palette, while process color suits photographic reproduction.
Use spot color for logos, type, flat illustration, team wear and most textile prints with a limited palette.
CMYK process separation
CMYK, or four-color process, splits the image into cyan, magenta, yellow and black. Each is printed as a halftone at its own screen angle, and the overlapping dots mix optically into a full range of colors. It is the method behind magazines and most offset printing, and in screen printing it can reproduce photographs with only four screens.
The catch for apparel is that process inks are transparent. On a white or light shirt they work well. On a dark shirt the garment color shows through, and the print looks washed out unless the separation adds a white underbase and extra highlight screens. Process printing also demands tight registration, good screen tension and higher mesh counts.
Index color separation
Index separation reduces the design to a fixed palette of inks, then describes tone with tiny square dots that are all the same size, scattered in a random pattern called diffusion dither. Unlike halftones, the dots sit next to each other rather than on top of each other.
Scott Fresener's Color Separations 101 describes index as working on both light and dark garments and being forgiving on press because every dot is the same size, though it typically needs more colors than simulated or process methods. His index guide puts the usual palette at six to eight colors.
Simulated process separation
Simulated process, sometimes called fake process, uses opaque spot inks rather than CMYK, printed as halftones that blend into photographic tones. A typical job prints a white underbase first, then a set of spot colors chosen from the artwork, such as a red, a yellow, a blue and a gray, often finished with a highlight white.
Because the inks are opaque, simulated process works on dark garments as well as light ones, which is why it is the standard for detailed, photo-like prints on black shirts. It is also the hardest to separate well by hand, and the main reason dedicated separation plugins exist.
The four methods side by side
| Method | Inks | Dots | Garment | Best for |
|---|---|---|---|---|
| Spot color | One premixed ink per color | Mostly solid, halftone for tints | Light or dark (with underbase) | Logos, type, flat art, textile prints |
| CMYK process | Cyan, magenta, yellow, black | Overlapping halftones at different angles | White and light | Photos on light garments, offset print |
| Index | Fixed palette, often 6 to 8 | Same-size square dots, side by side | Light or dark | Tonal art where an even, forgiving dot helps |
| Simulated process | Opaque spot inks plus white | Halftones, usually over an underbase | Dark and light | Photo-real prints on dark shirts |
Spot color vs process: how to choose
Most choices come down to three questions.
- Is the art flat or tonal? Flat shapes with clear edges point to spot color. Photographs, airbrush shading and soft gradients point to process, index or simulated process.
- What color is the garment or fabric? CMYK is at its best on white. For dark garments, choose spot color with an underbase or simulated process.
- How many colors can the job afford? If a flat design has eight colors, try reducing it to four or five before reaching for process. If a photo needs to be printed, four process screens may be cheaper than eight spot screens.
For textile designers sending repeats to a mill, the same logic applies to rotary and flatbed screens: each color is a screen, and fewer, cleaner colors give a more reliable print.
Where separation happens in practice
Separations are made in graphics software, either by hand or with an automated tool. Photoshop can build any of the four methods with channels, and our tutorial on color separations in Photoshop shows the spot color workflow step by step. Plugins such as T-Seps and UltraSeps automate simulated process and index work; we compare them in our roundup of the best color separation software for screen printing.
For spot color, Texloom's Separation Studio separates a design automatically, or into the exact number of screens you choose, adds trapping, registration marks and an optional white underbase, and names each plate after its nearest color in the Texloom Color Library. It is a spot color tool, so it does not make CMYK, index or simulated process separations.
Next step: read the color separation pillar guide for halftone settings and ink naming, then separate one of your own designs into spot screens and count how many you really need.
