Hi Calvin,
Uniforms colours are tricky as they start to wear and fade as soon as they are issued. By the time the uniform sees action it may have changed colour dramatically depending on locale.
I am by no means an expert but, alot of uniform fabrics are dyed cottons and twills and won't hold the original colour for very long once out in the open. The salt from the sea in the PTO would be enough to change the colour let alone in combination with the sun.
I would look for reference photos that show groups of soldiers together and you'll see what I mean. Some guys will have newly issued pants ( which would be a darker colour) and their original combat jackets (which would be lighter) . Unless the soldiers are fresh from the recruiting depot there will be some differences in uniform colours, very noticeable in black and white.
From some of the pictures I have seen of marines in the PTO the uniforms seem to loose the colour and fade into a very light green grey.
For Marines I would start with a Vallejo Panzer Aces series colour called...wait for it..." USMC Tank Crew"... it looks a touch dark to me but I would use it as a base coat. For the actual colour of the uniform I would use Vallejos USMC Tank Crew highlight, which looks closer to the twill green that the cloth should be. For highlights use the lighter of the greens with a bit of white and just hit the highest points of the uniforms, collar edges, pocket edges, epaulettes things like that.
If your just getting into painting figures there are some great figure painter sites on the net, just write me and I can forward some sites.
Over on Hyperscale they have been showing pictures from Life magazines photo archives. There are quite a few photos in colour.
If you go over to Lifes site, do a bit of a search ( US Marines? or US Army) you will come across some great reference photos.
Research is one of the best parts of our hobby, I for one can say I'm always learning and hope to continue to do so.
Hope this helps, good luck, and if you need any other help just write.
Joker