Siamese rats/mice molt out of their dark colouring, Tom. Humidity can also effect the pattern, and successfulness of their molt. Which is why some siamese rats/mice just turn out fugly.

Reverse siamese rats do not have or need siamese genes, at all. They are simply poorly bred (or well bred, if you were going for reverse siamese I guess) normal colours, that do not have even saturation of colour, and have light points due to bad (or again, maybe good?) breeding of a colour.