what should the pre-shade colour be on desert vehicles (especially the M2A2) and what should the main coat be?

You can use flat black or flat brown and Gino confirms that Model Master Sand is the closest match to the real color.

I confirm what Pete says. Model Master Sand, FS# 33531 is the closest match. Good luck.

thanks man.....i saw a French Leclerc in its desert scheme and i saw that the pre-shade colour was NATO Brown....will that work to?

NATO Brown will probably work as well. I personally don't do any pre-shading. I don't think it looks realistic; artistic yes, but not realistic.

its true what you say. From photo's that i have seen of vehicles in iraq, there is no pre-shading. Its just the one main colour.
But by pre-shading it gives it a more defined look.

Similar to Gino, i dont like doing pre shading either. For enhancing gaps etc, i prefer dark washes.

For the desert paint, if you are like me and cant get access to the Testors range, try this:

Desert Yellow - 2 part
Flat White - 1 part
Buff - 1 part
Flat Yellow- 1 part

US Sand colored vehicles are all one color, the above Testors MM Sand. There is no pre-shading or second color on them. What you are seeing is shadows and /or dirty areas. Like Jazza, I prefer to use washes and weathering to get a realistic effect on my models.

on my first M113A2, i didn't use any pre-shading. Most of the things i did to make it look realistic was weathering.but i dont really know how to do the washes? can you please explain?

Check out the Features section above. There are lots of pointers there.
For example:

Washing and Drybrushing Armor