Chinchillas do change color over time...Beiges will turn more orange, Pink eared chinchillas will get freckles all over their ears, mosaics will darken and fade during the first year and a half, Black and Brown Velvets will continue to darken and get more veiling down their bodies the first year.....
But looking at your pictures, they don't get a spot that big in 7 days. It looks to me like your white has come into contact with something that has stained his fur. Look around the cage and see what might be getting on him.