Do you require a cmyk image file - a jpeg or a psd There is a plugin called Cyan which is useful. there is an example here: https://www.gimp-forum.net/Thread-gimp-2...8#pid30238
Gimp is a RBG editor and all editing is RGB.
You can separate a rgb image into its c-m-y-k equivalent layers using Colors -> Components -> Decompose and choose CMYK {or RGB if that is what you want) from the drop down list.
That gives you greyscale images which are the masks of the separated colors.
It is a big subject and you should start with general concepts such as https://www.lifewire.com/color-separation-1697622
As you see, it is mostly a PhotoShop subject.
Edit: Just looking at your other posts: Is this for screen printing ? Some software mentioned here https://silverbobbin.com/color-separatio...-printing/
I can think of ways in Gimp but how many colours are you considering? Each colour requires its own black & white mask.
Gimp is a RBG editor and all editing is RGB.
You can separate a rgb image into its c-m-y-k equivalent layers using Colors -> Components -> Decompose and choose CMYK {or RGB if that is what you want) from the drop down list.
That gives you greyscale images which are the masks of the separated colors.
It is a big subject and you should start with general concepts such as https://www.lifewire.com/color-separation-1697622
As you see, it is mostly a PhotoShop subject.
Edit: Just looking at your other posts: Is this for screen printing ? Some software mentioned here https://silverbobbin.com/color-separatio...-printing/
I can think of ways in Gimp but how many colours are you considering? Each colour requires its own black & white mask.