I use craft acrylics that I purchase in craft or art supply stores (or online from Amazon). They are very inexpensive and last a long, long time. (Of course, more expensive acrylics are available, too). There are hundreds of colors, and you can mix your own colors, too.
The higher the quality of brushes, the more rewarding it will be to use them.

I also use Posca paint pens. Only Posca. There are just no other paint pens that work as well, or last as long.

I prefer to use primer - Liquitex Gesso (either white or clear, depending upon the rock and the result I want).

To seal the rock I use a coat of Mod Podge glossy (because it doesn't smear colors). When that dries I do a couple of coats of Liquitex High Gloss varnish - makes the rocks very shiny and weather-resistant.

Oh - and all sorts of glitter is fun and make the images really special!

So, this is my method. Everyone experiments and uses what works for them.