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Basically I want to mix halftoned spot colors in the way you do CMYK, but with any combination of spot colors of my choice.

Photoshop has an almost-good-enough way of doing this with image>mode>indexed color, where you can specify a limited set of colors and each pixel can only take on one of those colors. You can separate the colors easily from there with the magic wand tool, but the pixel dithering effect obviously prevents you from being able to make good halftones with those separations.

sample image sample image

sample image with indexed color sample image with indexed colors: white(ish), green, blue, black

I'm under the impression that some specialty software for t-shirt screenprinters does this, but I'm hoping theres a way to do it with adobe apps.

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This is for Adobe Photoshop

  1. Convert the RGB image to CMYK by doing Image > Mode > CMYK

  2. In the channels panel, delete the magenta channel. You could even delete the black channel if you want, since it's not really contributing much. I think this would print just as well with two colours - Pantone Process Cyan + Pantone Process Yellow. Of course you might want to experiment with other shades of blue or yellow.

  3. You can change the colour of the remaining channels to spot colours by double clicking a channel thumbnail, click on the Color square in the Spot Channel Options, and hit the Color Libraries button to bring up the spot colour libraries.

  4. Finally save as PSD, and send this to your screen printer.

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Example for 3 colour printing enter image description here

Example for 2 colour printing enter image description here

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If the original design was not intended as a spot ink, your work can potentially not be optimal.

Sayed that, one general idea is that using grayscale images for each ink, and using a blending mode -> Multiply will give you a good representation of the final product.

One method of separating colors from grayscale plates is taking your existing channels as a starting point.

Let me give you some posts where I describe a way to do it.

Preparing design for duotone printing?

Printing photographs when job is a 2 spot color job

Your method of using indexed colors sounds interesting. The only drawback is that you will have a strong grainy look due to the dithered nature of the separation, and some gradients will look too harsh. You need a really high-resolution initial file in order to have smaller dots, but it is an interesting option.

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  • I'm learning a ton from your past answers, thank you for sharing all this! Nov 8, 2022 at 19:42

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