Our neighbor in Louisiana had a "late 70's" van conversion with the big BIG side windows. The carpet when it was new was a really nice medium brown. When he traded it in around the mid 80's, the carpet was a really REALLY ugly lime green/yellow "schmutz". It was discolored/faded from the sun even under the seats where the sun should never have shined...... Reflected UV rays, like water and odor, tend to go where they want, not necessarily always in a straight line.....
As for changing color with age.... Hmmmmmm "no comment"