I'm with those who say go dark to light. As for paints themselves I prefer the Delta Ceramcoat paints that every craft store but Michaels seems to carry. They have a much better pigmentation and are quite smoth, though they do cost about a quarter more per bottle. They are the craft paint that is closest to miniature paint and I use them on my minis as well
You might like this easy formula I used for this cathedral that would be good for a desert rock.
Full Article here:
http://chicagoskirmish.blogspot.com/2014/09/terrain-cathedral-de-la-pegasus.html
Start with a dark brown (Burnt Umber is a good one) then a wetbrush of Terra Cotta (deliberately not painting all the cracks so some of the brown shows through. Then followed by a light drybrush of Bamboo. It's three quick steps that will quickly give a nice mottled medium brown appearance. I let a fair amount of the undercoat still show through, but you can always apply the terra cotta and bamboo a touch heavier to make it look a bit less dirty.
For a bit more depth, try the formula used for this piece.
Full article here:
http://chicagoskirmish.blogspot.com/2015/06/dragons-dont-share-but-mike-does.html
Black Gesso base/prime, Sprinkle on just a bit of sand here and there for some more texture, Wetbrush a mix of Burnt Umber and Red Brick (not sure the exact color), Wetbrush with Terra Cotta, Drybrush Trail Tan and Drybrush of Bamboo.