How long is the closet light on? Any type of light -- not just sunlight -- fades fabric. This includes flourescent lighting.
If the industry standard for fading is 20 hours, it may be that the time the closet is light is on is adding up over time. If, for example, the light is on for 10 minutes a day 6 days a week, the 20 hours would be up in less than half a year.
Some dyes are more colorfast than others, so there is quite a variation in colorfastness among colors. Some are much better than others. Sounds like greens and blues may be among the least resistant to fading.