From the photos I've seen the green gun's are green, not grey or black. Tamiya nato green looks about right.
I'm trying to find a good colour match for mine too. White ensign models makes a colour called Gulf Armour Light Stone (BS 361), their website:
http://www.whiteensignmodels.com/.The British colour is quite yellow, rather than sand. I was thinking of mixing a little Tamiya flat yellow with either dark yellow or desert yellow.
If you go to Yahoo, and click the image tab in their search feature, type in 'Challenger 2 Iraq' or something similar it will bring up plenty of good photos. Yahoo worked better for me than google.
Here is the MOD site that has large photos:
http://www.operations.mod.uk/telic/photo_gallery.htmHere is the MOD search able photo database:
http://www.defenceimagedatabase.mod.uk/fotoweb/this one only has small photos on it though.
Hope this is of some help. Let us know what colour you decide to use.