CMY color decomposition for printing with GIMP

I am currently (may of 2020) in the process of self publishing the collection of my best BJJ themed illustrations and comics:

This project is going to be printed in risograph colors, so I needed to convert some of my illustrations in 3 different layers for the 3 colors: Yellow, Magenta and Cyan for the printer.

Note : I didn’t use a Black layer (the K in CMYK) for this.

Searching online I found that the CMY decompose function in Gimp doesn’t seem to work correctly, so I had to come up with a really simple trick in order to achieve the results I wanted.

Here’s the acrylic painting that is being used for this project, it’s a take on the iconic LOBO’s Back! comic cover (click here to see the original) with a BJJ twist:

So, in order to prepare the image for risograph printing, I’m going to use the Decompose function in the Colors Menu >> Components >> Decompose:

Next, I am going to choose RGB and Decompose to layers :

The result is a new image with 3 layers:

Here are the 3 layers put side to side just so you can see the result (in reality they are on top of each other):

GIMP has given those layers the RGB colors automatically:

The one on the left has been called RED
The one in the middle is called GREEN
The one to the right is called BLUE

Now, to understand how the color printing works, the darker the area, the more color will be applied, and then by superposition the initial colors will appear in the printing, but… As you can see, if we use those layers like that it doesn’t work: the painting’s background is orange, so there should be a lot or yellow and some red, but here it’s the blue layer that is the darker one…

The trick is to simply rename the layers like this:

The BLUE layer will be used for the YELLOW ink
The GREEN layer will be used for the MAGENTA ink
The RED layer will be used for the CYAN ink

I just sent those layers to the risograph printer for the test prints of my projects, and here is the result:

The colors aren’t exactly the same because : 1 – it’s risograph, and 2 – I just used standard Yellow, Red and Blue inks instead of searching for Magenta and Cyan ones, I suppose if I did so the result would be better. But here you go, easy CMY color decomposition using the RGB function and simply renaming the layers.