I use a mix of Reaper Master Series and Vallejo Model Air for my Raptors. This is the color progression I use, from darkest to lightest, with black primer:
RMS Olive Shadow
RMS Olive Green
RMS Worn Olive
VMA US Light Grey
I'm not really a fan of VMA Olive Drab because I find it a little too brown for my liking. Since I prefer my Raptors to be more green, I used VMA Olive Grey until I switched to RMS paints. Here is a color swatch I made for someone comparing all the olive drab colors I have (VMA US Light Grey is missing from this chart because it isn't a "drab" color... it is a light grey-green):