That's always my first thought but the fade is just prevalent away from the windows. Usually I see more of a difference in the less exposed areas. Just wondering if some nylon carpets are more susceptible to UV than others.UV, look at all those windows.
Could be ozone or some other fume fading.In my climate, The Blue dye is the first color to leave the carpet...
There is Sunfade from direct sunlight and what we call Ozone fading from whatever the air can get too... It can happen within the first 3 years..