Yellowing in alkyd films?

Question:i understand that the oxidation of one of the double bonds on the oil portion of the resin results in yellowing - however, is this yellowing attributed to a shift in light absorption due to the addition of oxygen to the polymer or is it a by product of a reaction? also, ive read that yellowing is reversible by exposure to light - how so? is it a "visual" reversal or an actual return of the polymer to it's pre-oxidative state?

Answers:
I used to work for a paint distributor, and we were told that the yellowing was a byproduct of a chemical reaction. UV light breaks down the byproduct, but not a reversal. The overall effect being that UV light has a negative impact on the product. Exterior paint is formulated to increase the chemical reaction so the byproduct absorbs the UV instead of other components of the paint, thus extending useful life. Sorry, I can't remember the details of the reactions -- didn't really seem important at the time.