Hmmm, this is a little tough because it really depends.
You might want to determine what exactly the colorist will be working on. Will it be just backgrounds or will it be just characters? Or will it be both? That could change up the price depending on the amount of work they take.
And based on some convos that I've tuned into in the past, a fair price for a BACKGROUND colorist for a webcomic would be about 3 USD to 5 USD per panel. However it's a generalization.
Personally for me, I have a background colorist who helps me out from time to time, and the prices that I have for them are actually based on what they want. We negotiated. So, I pay 4 USD to them per panel.
I would advise asking other around people across other social platforms about what they feel would be the most fair price and maybe also discuss it with your potential colorist(s) and see how they feel is most fair to them!
This wasn't exactly the most helpful but I hope you make some use out of it ^-^ !