Mike: There's a bunch of people around here much more experienced with this technique than I but since no one else has posted I'll take a stab at it. 

Most tutorials I've seen they seem to use primary colours like red, blue, green etc- I'm not sure if there's any rhyme or reason to it other than artistic 'feel'.

I'm a little more conservative, I've done a few and just use more 'earthy' colours like grass green, dirt brown, and sandy tan. 

A word of advice. Don't use a green oil paint over a dark yellow or desert sand scheme. You will get a disturbing BLUE color that's hard to remove even when wet. Mostly on armor I use white, raw umber, and black as dot filter color. At times I use red too, but don't use much as it will make your armor look very rusted. Most armor modelers think rust should be kept to a minimum.

Thanks Wood, I've only done US olive drab, Soviet dark green, and Japanese three-colour schemes so far. I'll keep your advice in mind next time I do a sand scheme AFV. 

Never thought of white, raw umber, and black, will have to give that a try- thanks! 

Oh, you have a good question.

Gamera and Wood are both correct. But basically, the reason you don't see anyone list a chart of what colors to use are because it is an artistic choice.
For green vehicles you want to use green tones, and maybe a touch of blue here and there, but mostly you want to use earthy tones of browns. Toss in a dash of yellow, white, black, and blue, but stick mostly to green and browns.

For a desert scheme you'll want to go with mostly tan tones of yellows and ochres. Tossing in  a dash of white and brown here and there to break up the monochomatic scheme.

I bet you're seeing a pattern here. You basically want to have random mixed tones of the theatre that the vehicles are operating in.

So yeah, Gamera and Wood are both correct. Follow their advice. :)

oh, and for armor, you'll want to blend the oils over a semi-gloss clear cote. Gloss works better, but it is sometimes more difficult to dull the sheen back down to completely matte. This is just my personal experience and may not reflect everyone else's experience. However, using the oils over a flat sheen will give you less blending. The colors will be stronger and blend a lot less. You can actually use that to your advantage if you want such an effect as a dirt build-up where mud may get ground into a painted surface.

So you will want to paint a scrap of sheet plastic or cardboard, semi-gloss half of it and make half matte, and play with your oils. You'll get the hang of it real quick.

This technique is not tricky and can be mastered in a reletively short amount of time. So go have fun!  :)

Thanks Mike, you explained it much better than I could. 

first take advantage of craft store 40% coupons to stock up on colors. you don't need professional but go with artist instead of student.

second get really good brushes and those cheap plastic artist pallets.

i use primarily raw umber. black, payne gray, and white. i have used prussian blue to create an "WOW IT'S REALLY COLD" LOOK." and yellows and reds on various panels. a jungle scene may see some green. and you can mix paints together so you might not use green but may "adjust" a gray with some green for a new color.

get some sheet plastic, prime and primary color amd filter various sections and see how it looks. and if you thing whatever you have domne on a model might beenough it is.