Siamese kittens are born white because in their mother’s womb they have experienced an ambient temperature which is even throughout their body, internally and externally, and which is warm at 99.5º-102.5º Fahrenheit which is about 38º Celsius. The Siamese cat coat is temperature sensitive. Where the body is warm it is white (or whiter) and where it is cooler it is darker.
The sub-adult and adult Siamese cat has dark extremities because the extremities of the body have cooled and are permanently cooler than the core areas. As stated, where the body is cooler the hair is darker and vice versa.
When the kitten has been born the extremities of her body will cool down while the core of the body at the surface i.e. the coat at the flanks will maintain a temperature nearer to the core temperature and therefore remain lighter coloured.
As a cat ages the blood circulation deteriorates which causes the flanks of the body to become cooler and nearer to the temperature at the extremities (tail, face and end of the legs) which results in the white areas to darken.
You’ll see some Siamese cats with quite dark areas of fur all over the body with even darker extremities.
It is all about the production of the pigment melanin in the cells of the skin and which ends up in the hair strands being temperature dependant in Siamese cats and other pointed cats. Through selective breeding resulting in the introduction of specific genes breeders have changed the color of the darker areas from dark brown (seal) to other colors such as gray (blue) and lilac.