A lot of people wonder what they should use to paint the rocks. Acrylic paint is definitely the best choice. You can either use acrylic paints with brushes, or acrylic markers. I prefer acrylic markers because they give me more control. But many people prefer paintung with brushes. 

Since I almost exclusively use acrylic markers, those are the ones I can make recommendations about. 

WHAT TO AVOID

There are some painting supplies that just won't work well for rock painting.
Tempera Paints do not work AT ALL! They will wash right off the rocks if they get wet, even if you try to seal them.

Alcohol markers are awesome for blending colors, but the colors fade very quickly, even if you use a UV resistant sealant

Regular alcohol inks may not fade. Do an experiment to find out... paint ine rock with alcohol inks and leave it in the sun for at least a month!