May I suggest, for the granite, a technique that works for me once, and following on from what Monica suggested, sponging layers that are quite translucent. If you use acrylic paint (with at least 50% clear medium), and a small natural sponge, you can build up a complex surface of many greys and blue greys. This technique, known as glazing, in the art world that I hail from, works a treat, and allows you to make adjustments to colours on the fly. Rooney system 3 and their glazing medium work perfectly for this, and I fooled a lot of folks with a chunk of styrofoam once! The natural sponge, although pricey, does give a more organic look, and will replicate the complex surface of granite really well.