i would get some acryilc paint from walmart in the craft section. You will need some black, gray, burnt umber, burnt sienna, Dilute the paints with water, probably like 9-1
You can use spray bottles if you have them, they work really good.
Otherwise just make sure the paints are pretty watered down, and start at the top obviously and you can use a foam brush.
Start with some black/gray then switch off to some burnt umber/sienna, and just keep switching back and fourth, until it gives you the look your looking for.
You really cant mess up as long as you have a nice base coat of the black/gray on your rocks first.
There is a bunch of good how-to videos on youtube, i will try digging some up for you.
Heres the first one i ever did, not really knowing anything, and just winging it, its good to have some practice
After doing one, you kinda know how to do the next one better, and what color to use more of and what to use less of, no biggie really, as long as it looks good to you thats all that matters