Their fur problem is just molting. At 8 weeks babies will shed their old fur and start getting their adult coloring. They can go through a dramatic change in this time period. Some rats start out of beige and then turn into siamese etc. Adults usually molt once every three months or so. However their problem could be dinginess in color. Some poorly bred rats color is not as good as it should be. Although it sounds as if they were molting.

Rex and velveteens fur will sometimes appear to be different shades as the sun hits it from different angles. Rex and velveteens are curly whiskered and curly furred rats whose undercoat is sparse. The undercoat of velveteens is missing all together making them very very soft. Since their fur is curly it tends to seem like they have three different colors.