All of the Siamese I have produced start out light golden colored up front and dark golden/brown in the back half.
They have the dark areas right from the get go, the coat lightens up and the points tend to darken and stand out more as the Siamese rat ages.
They never really are white, as adults the good ones stay dark on the points and turn a light creamy color.
If you're ever wondering the color just hold it up next to a hooded rat to compare and see if it truly white.
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Siamese is a recessive trait so both parents would have to either be a Siamese or carry the Siamese gene to produce more Siamese. I see what looks like beige in the dried off pic but you will know for sure at weanling size when it molts.