AS ever It depends!
AS90's are NATO Green/Black for UK & Europe. You will need the 39 calibre Short barrel version, as the UK does not deploy the 52 calibre barrel, often called "Braveheart". That name is pure..... marketing 
SOME refs say the AS90 missed the 1st GW, I honestly can't remember, someone will 'know'?
When repainted, every inch would have been painted, not just the top, & the GW pics I've seen indicate lots of filth, they weren't showground queens.
My B-I-L, who commanded a re-supply for the Arty in GW1, says that sometimes his guns (M109-A2's) were ahead of the Challengers, as once a column of moving Chollys were firing THROUGH his (moving) supply trucks to hit moving Iraqi tanks.....
The 1990/91 Gulf War 1 Operation Granby Colours were Light Stone painted over the top. this was known to be fragile, weathered heavily & was quite pale, such as Model Master British Gulf Armor Light Stone
The 2003 GW 2 Operation Telic was a harder more durable & distinctly yellower colour. I will look for a Yellow sand colour here. Someone will be able to give a colour ref here?
A Build blog here: http://www.militarymodelling.com/forums/postings.asp?th=19599
Info page: http://www.onepointed.com/dan/model/ModelLinks.shtml