Now, I’m speaking of this as an artist.
For the love of God, don’t use the entire damn rainbow in your character design.
When you ask for a request of your OC that looks like it came out of a bag of Skittles, it’s hell for the artist you requested to colour that thing.
When you wanna make a comic of said character, whether it be a backstory, or just a simple story you wanna tell through that character, God help you and your motivation to finish it.
Trust me, I’ve been there. I’d rather have my character nice and plain than to have a character with the whole colour wheel in their palette.
Five colours with only one of them contrasting the rest is more than enough.
Or stick to three hues at best.
With one of my characters, I stuck to a warm palette of reds, browns, and oranges.
You can make the colours contrast a bit by making the fur of a furry OC a light blue, for example, but have a complimentary colour like Orange, or any other colour that contrasts nicely, like Red.
Get creative, but seriously. Don’t use the full spectrum of colours in one character. It’s really gonna be a pain in the long run.