Shiny Sprite Help

I was wondering if anyone had any tips on changing the in-game sprites. So far, I haven’t been able to find a program or website that lets me alter the sprites pixel-by-pixel or even just by colors. I’ve been wanting to make some rainbow versions of sprites so I would really appreciate it if someone could recommend a program and possibly a request. Thanks!

personally i used gimp doesnt really matter what program u use as long as it has an option to paint 1px
paint wont work because it does something like this
depending on the brush that u choose