I can give you Humbrol colours that are pretty close. 121 light stone (or pale stone, can't quite remember the name) gives you a good colour for the over all sand colour. 120 light green, mixed with a little white as required gives the faded looking light green colour, and 170 (that's a dark brown) mixed with a small amount of white gives the 'coffee stain' Brown areas. Hope this is of help.
You wont find FS colour codes for paint colours for uniforms because the Militarty don't paint uniforms! The best way is to search the web for lots of photos, and mix up your own paints that look close. You could try www.uscav.com they sell military uniforms and you might find photos there, or www.ebay.com and search for desert cammo or something like that, you can find a lot of people selling uniform items, usually with photos, and these are often second hand and therefore more like the items as they are found in the field.
hope this helps
Phil