I think evalotion has a key role in this question. I have done some research and have found many sites saying that in the summer the reason a dogs coat becomes lighter and thinner in color and texture is so it does not absorb the sun light hence making the dog cooler and of course the opposite for the winter the thicker coat keeps them warmer and the darker color absorbes the heat from the sun.

Since I am a mommy to a Bichon I do not have the shedding problem so his coat never changes