While speaking to engineers from Michelin, Goodyear, Bridgestone and a few chemical companies that manufacture dressings and cleaners for popular consumer/pro lines, most told me that tire browning occurs for one of a few reasons:
1) The silicone reacts with moisture and turns brown
2) The additives in the tire migrate to the surface of the sidewall and react with moisture and turn brown
They indicated that in very humid climates, it's not uncommon for a dressing to brown just from the humidity in the air... I thought that was pretty interesting.
The amount of "gloss in a dressing" usually has little to do with dirt attraction - I've seen some really matte products attract their fair share of dirt/dust. But, as we all know, everyone's results may (and usually do) vary.
rj is 100% accurate about the cleaner exaggerating the browning effect - some strong cleaners can force the tire additives to the surface, and the reaction is evidenced by the brown foam that forms on the tires when wet.