Dave Frary has instructions on using his rock molds ..
Using Your Blue Ribbon Rock Molds
I use a different process then Dave does but followed his recipe for making up a basic earth color. You mix 1/3 of your latex paint and the rest water. The paint? I just had the cheapest flat latex mixed up at Lowes. I picked out what looked like a yellowish / tarnish color. No biggie .. just don’t make it too dark. Me .. I darken mine as it goes. You can use almost any light color you wish since you will be staining later.
On an aside: That’s a ‘Chip Brush’ I am using. I picked up a box of them at Harbor Freight ‘on the cheap’. Great for stuff like this.