Mine's a novel too but I've spent a lot of time working on colors for Will and Crow. I visualize the character sometimes based on instinct but these two specifically needed to have an image that harmonized with each other. So I gave each a form of limitations:
Both are around the same height, Crow just being 2 or so inches taller
Will: the non traditional skin tone for a 'bottom', he's tanner and darker than Crow. Rich clothes but smartly dressed with vests and the like. Pink hair that's a dusty sunshine shade, blue eyes that match a clean ocean. Usually wears earth tones, but can be seen in leaf greens.
Crow: paler than Will, with a yellow tone to his skin. Gold eyes, like actual real metal gold. Poorer, so more variety to his clothes and colors but primarily in strong shades -strong reds usually.
Here's a picture of their initial outfits I designed for them
While Will's in colors, his colors are just a tad washed out compared to Crow's. A pale yellow shirt, a dusty navy vest, and black slacks. Crow's in strong red, and tan pants (not an outfit he wears often). Even when I wash out of the colors with a different coloring style, the rules still applies. (art by my friend caim-the-order on dA)
I also recall spending like 2 hrs finding the PERFECT shade of pink for Will's hair. It's not too red, not too orange, not to bright, not too dull, not too purple, Oh my god it took forever. I never want to relive that.
Heck, even more recent art shows that I still follow the rules. Crow's in bright colors, and Will's in dull shades. (Crow does wear stuff besides burgandy and red lol he thinks he looks sharp in royal blue and pumpking orange)
https://www.deviantart.com/watashiveracasan/art/Anniversary-3rd-Year-488976991