I don't understand your need for a lightweight, four-season tent that is tan--unless you're in the special forces or are planning on stealth camping in the desert.

But given your requirements, you should go as soon as possible to the Stephenson's Warmlite web site and choose your color.

Of course, I have seen tents in the color you describe--I even have one: a Bibler Bomshelter made for the US special forces. Its main body is a light tannish green, and the vestibules are the standard Bibler light grey. And although they are not lightweight, on eBay you can find tents made for the USMC by TNF and Eureka that have desert camo rain flies.

As for UV degradation, unless you're camping for months at a time in the desert or you're above 15,000 feet, you're more likely to have the zippers fail or the coating peel off the floor before sunlight does much damage to your tent.

Polyester (instead of nylon) rain flies have been used on MEC and TNF expedition tents, and they are more resistant to UV damage and stretching when wet than traditional nylon flies. The downside? Polyester has a lower tear strength and less abrasion resistance compared to nylon. Having said that, I like polyester flies better than nylon ones.