You have two options. The first is marbling, as described above, though it takes a fair amount of skill and time to pull off. I would note they did not use this effect on the necron vehicles. Look closely, they are a lot flatter than the overlords shoulderpads.
The second is stippling, either with a brush or a sponge, as seen in this picture. I used a sponge on the model below, starting with a green base, then sponging on two lighter colors over the top.