That is the effect of the acromelanism (aka siamese)
The acromelanism simply means it is temperature dependent. The warmer the areas, the less pigment. The colder extremities are, the darker, like nose, bum, feet and ears. (Assuming the animal is not marked in some manner) Not to be confused with Agouti. If it were an agouti siamese, he would be very light and not look much like a siamese.
Anyways, I'm quite certain you do not have Agouti's. It's quite noticeable.