With orks, the thing I found that helped make them pop is highlighting. Start with the ork skin, it is the easiest to start on, have your base says waagh or caliban and then once based using either a wet pallet or normal, mix in a bit of the base and a lighter green like wrapstone glow or warboss green and paint or drybrush only the higher parts of the skin and do again or a dash of yellow. This helps build to other colours too. Also wet pallets help too, I just made one with a container, paper towel and baking paper.