How to Make Index Color Separations for Screen Printing (Step-by-Step Guide)
A complete workflow with setup, palette creation, channels, underbases & output
Index color separation is one of the most reliable methods for preparing multi-color photorealistic designs for screen printing—especially for textile printing. Instead of halftone dot size variation, index separations use a fixed-size square dot pattern (random diffusion dither) and blend colors through pixel distribution rather than gradient dot growth.
The result?
🟩 Consistent prints
🟩 Minimal moiré
🟩 Easy press setup
🟩 Excellent color accuracy & repeatability
What is Index Color Separation?
Index color reduces an RGB image—with potentially millions of tones—into a limited color palette selected manually. Photoshop then converts the artwork into a randomized diffusion dither pixel grid, where each dot is the same size (FM/Stochastic screening).
📌 Unlike CMYK halftone printing, dots sit beside each other rather than stacking, which makes printing more forgiving and consistent.
When to Use Index Color
Index color is ideal for prints requiring high detail, texture, skin tones, and complex imagery—while maintaining control on press.
Best suited for:
| Use Case | Why Index Works |
|---|---|
| Photorealistic prints | Soft tonal transitions and detail retention |
| Dark shirts | Works with underbase + highlight white |
| Medium-to-high color complexity | Accurate palette control |
| Shops using 6+ presses | 6–10 colors recommended |
Limitations to consider:
- <6 colors → image may look posterized
- 4-color presses = only suitable for simple artwork
- Revisions require re-indexing the entire file
Key Characteristics of Index Separations
| Characteristic | Description |
|---|---|
| Dot Type | Random diffusion-dithered square pixels |
| Screen Type | FM/Stochastic – all dots identical size |
| Color Mixing | Optical blend → dots placed side by side |
| Typical Color Count | 6–8 (sometimes up to 10 for realism) |
| Press Requirement | Minimum 6-color recommended |
Step-by-Step: Creating Index Separations in Photoshop
Tested up to and including Photoshop version 26.8.
A) Prepare the Image
- Open image in CMYK — never CMYK (check Image > Mode/RGB Color)
- Resolution at final print size = final output DPI (cannot be changed later) (check Image > Image Size/Resolution)
- Standard: 180–200 dpi
- Detailed work: 200–225 dpi
- Adjust tone curve: (Image > Adjustments > Curves/Input select button left line to ≈10)
- Optionally, apply an Unsharp Mask (Filters > Sharpen > Unsharp Mask) to reduce any softness caused by previous JPEG compression.
- Slightly oversaturate if colors appear weak (Image > Adjustments > Hue/Saturation)
- If heavy shadows → lighten black ranges (Image > Adjustments > Curves/Input select right top line to ≈250)
- Save as Design-Name-on-White.psd
B) Build the Underbase & Highlight White (for dark shirts)
- Create a Duplicate Version – Make a duplicate of the original RGB file (Design-Name-on-White.psd), Image > Dublicate
- Create a Masked File: Cover the area outside the design in black to represent how it will appear on a black garment.
- Use the Color Range as example, (Select > Color Range > ok and Edit > Fill > Contents/Black (Opacity 100%))
- Check that Image is Flatten, Layers > Flatten Layers.
- Save as Design-Name-on-Black.psd).
- Create Highlight White
- In the Design-Name-on-Black.psd . Select just the white in the design (Select > Color Range). Use the Fuzziness slider to determine how much white you want. Make sure Invert is checked.
- Save this selection as a channel (Select > Save Selection). Name this channel Highlight White.
- Create a Underbase
- Keep the file open with your highlight white in it (Design-Name-on-Black.psd), select > deselect.
- Select just the black in the design (Select > Color Range). Make sure Invert is unchecked!
- Select > Save Selection, as underbase.
- Select > deselect.
- Select the underbase channel only and adjust, Image > Curves to your needs.
- Output Highlight and UnderBase
- Spilt this channels, Design-Name-on-Black.psd, in the channel Panel, click right top bars, Split Channels.
- Delete the RGB Cahnnels
- Select the Highlight White layer, then go to Image > Mode > Bitmap.
Make sure the Input and Output Resolution are set to the same value, and choose Diffusion Dither as the method. Click OK to confirm. - Do the same for the Underbase file.
C) Color Separation for Indexed Color
- Open Design-Name-on-White.psd file
- Convert file to Index, Image → Mode → Indexed Color
- Uncheck Preview!
- Set Palette = Custom. In popup Panel, select with left mouse all colors queres, let go and in the new popup panel select 100% white. Hit OK and repeat OK.
- Now that all squares are white, you can start assigning colors for your design. Click on the first square and use the Eyedropper to select the color you want. Repeat this process for each color in the design. Including black, but not white. The more colors you select, the better the result will be. When you’re done, click OK.
- Under Options, Dither > Diffusion, Amount 100%
- Optional, images with many Spot colors, tick, Preserve Exact Colors.
- Optional, Changing Image > Mode > RGB, shows you how it will look like printed. Don’t forget to switch back to Image > Mode > Index Color.
- If you’re not happy with the result, start over and try selecting different colors.
- Create output channels, for each color, Select > Color Range > Fuzziness 0, Invert selected. ok.
- Select > Save Selection, give it a proper name or Pantone Number.
D) Preview Separation
- Keep Design-Name-on-White.psd open, In Channel Panel create a new channel, by clicking the plus + sign.
- Click in the Thumbnail image on channel, Give it a name like underbase, Color white Opacity 100%. Copy your underbase file in it, (from Phase B4)) select all > copy > past to underbase channel in Design-Name-on-White.psd.
- Do the same for the highlight white
- Do a new channel for the t-shirt color, select 100% black and move it under the underbase and highlight channels.
- Check the color channels to give it the proper setting as before, color name, color picker for color and 100%.
- More all channels in order as if you would print the later in screen printing.
E) Output File Separation
- You can split all channels again and save them as TIF and print them out on your film maker or DTS.
Many printers output underbase & highlight as halftone (55 LPI / 25°)
→ smoother gradients + better highlight transitions. Get more tips on Scott Fresener Website about Index Color Seps.
Mesh, Ink, and Print Setup
| Component | Recommendation |
|---|---|
| Mesh for color layers | 200–305 (70–120 T/cm) |
| Mesh for underbase white | 180–230 (70–90 T/cm) |
| Ink | Standard plastisol + high opacity white |
Rule of thumb:
200 dpi separation → use 280–305 mesh for best dot control.
Advanced Techniques
Convert Index → Halftone Channels
Useful when RIP is required, or smoother tonal transitions are needed.
- Convert Spot channels → 55 LPI / 25°
- Underbase & highlight maintain shape better
- Preserve CMYK-style smoothness while keeping index palette control
Moiré Prevention
Index prints rarely moiré—unless mesh count = DPI.
Avoid:
❌ 200 dpi index on 200 mesh
❌ oversized palette reduction
❌ scaling artwork after indexing
Match dpi to screen mesh correctly to maintain randomness and clarity.
Summary
Index color separations work like a mosaic made of fixed-size tiles: shading comes not from dot size but from dot placement and palette control. When executed correctly, the result is bright, sharp, highly repeatable prints with far less setup frustration than CMYK.
FAQ
How many colors do I need for photorealism?
6–8 minimum. 8–10 recommended for maximum detail.
Can I change colors later?
No — index = final. Changing a color requires full re-indexing.
Does index work on light and dark shirts?
Yes. Add underbase + highlight when printing dark garments.
