Colour printing costs more than black and white because it uses 4 inks (CMYK, or Cyan, Magenta, Yellow and BlacK) while black and white or greyscale just use black.
...It's only an issue if you print though, not for pdfs.
Putting something on Amazon/Comixology I guess is a straightforward option for what you want to do, but if you've made a pdf or ebook format, you can also just like... sell it yourself as a digital item. It might be worth doing some googling for "sell digital comics". Be sure to always check the legal stuff on a platform and their rules about the content being hosted elsewhere at the same time. I feel like I've seen stuff about Amazon being weird about it with novels.