I did an experiment to find out which color is the easiest to remove from sketches. I used red, green and blue because those are actually the basic ones used in image files. This is the original image that I scanned:
Now this is the red channel of the same image:
This is the green channel:
And this is the blue channel:
It seems that red is the better option, at least with the specific tones I have available, which is weird because everybody uses blue (at least that’s what I’ve heard).