Easter egg colours aren't especially waterproof or light fast, I would try a fabric dye if you are into handling toxic materials, but believe it or not kool-aid works as well, just not as well as the commercial products.
its not going to look good in the tank, it will get covered in algae/bacteria/whatever else is covering your rocks.
if its not going into the tank you could spend some time with simple paints, thinned out so they don't lay on thick, and really go at it. You could get the growth edge to look decent with some air brushing, maybe even find some fluorescing paints and spend way too much time putting dots on it with a tooth pick...