How to Create T-Shirt Color Separations in GIMP

This guide outlines the process of designing custom t-shirt artwork and creating accurate color separations using GIMP for screen printing. You will learn how to properly set up a print-ready canvas, isolate individual color layers, convert them into pure black separation films, and export them with registration marks to ensure precise screen alignment.

1. Set Up Your Canvas

Screen printing requires high-resolution files to avoid jagged edges and blurry prints.

Open GIMP and go to File > New.
Set your dimensions to the actual print size (for example, 12 x 16 inches).
Under Advanced Options, change the X and Y resolution to 300 pixels/in.
Set the Color space to RGB color and the background to transparent or your intended shirt color.

2. Design the Artwork

Keep your color palette limited when designing specifically for screen printing, as each color requires its own screen.

Create your artwork using dedicated layers for distinct design elements.
Avoid heavy gradients unless you plan to use halftones; flat, solid spot colors are the easiest to separate.
Ensure all lines and shapes are crisp to prevent bleeding during the screen exposure process.

3. Isolate the Color Separations

Once your design is finalized, you must separate each color onto its own layer.

Merge your artwork into a single visible layer if it is spread across multiple sub-layers (hide the t-shirt background layer first).
Select the Select by Color Tool (Shift + O).
Set the Threshold in the Tool Options to a low value (around 0 to 5) for solid colors.
Click on the first color you want to separate.
Go to Edit > Cut (Ctrl + X), create a new transparent layer, name it after that specific color (e.g., "Red Separation"), and go to Edit > Paste (Ctrl + V), then click the anchor icon to secure the paste.
Repeat this process for every distinct color in your design until all colors reside on separate, dedicated layers.

4. Convert Separations to Black Film Layers

Screen printing positives must be printed in opaque black ink on transparent film, regardless of the ink color used on the final garment.

Select one of your isolated color layers.
Check the Lock alpha channel box at the top of the Layers panel to preserve transparency.
Set your foreground color to pure black (#000000).
Drag the black color onto your canvas or use the Bucket Fill Tool to fill the artwork on that layer completely with black.
Repeat this for every separation layer. When hidden and viewed individually, each layer should be a solid black silhouette of that specific color's placement.

5. Add Registration Marks

Registration marks allow the printer to align every screen accurately on the printing press.

Create a new top layer named "Registration Marks".
Use the pencil or brush tool to place crosshair targets (a circle intersected by a plus sign) in at least three corners outside the main printable area.
These marks must remain visible and identical on every exported film layer.

6. Export the Print Films

Hide all color layers except the "Registration Marks" layer and the first separation layer (e.g., "Red Separation").
Go to File > Export As... and save the file in a lossless format, such as PNG or PDF.
Hide that separation layer, make the next one visible, and export again with a matching name (e.g., design_black_film.png, design_red_film.png).
Print each exported file onto transparent film at 100% scale for screen burning.