In the case of the Siamese, the coloring of the cats actually depends on the temperature in which they live.

When a Siamese kitten is born its fur is all white. As it grows, the plain coloring begins to change. Dark pigments appear on the nose, ears, tail and feet. These areas are known as points. As the Siamese matures, this color spreads and they have their prominent point color.

If the Siamese lives in a cold environment, another scenario will occur. This cat is also born white but will darken dramatically as it grows older. Instead of having a pale body with dark points, it will become dark all over, and sometimes may lack dark points all together.

The explanation of these differences is that in the Siamese cat, a lower temperature causes more dark coloration to be laid down in the growing hairs. This is why newborn kittens, warm from its mother's womb, are white all over. Then as they grow up in normal temperature, the hottest area of their body, around the stomach and back area, remains pale in color while their cooler extremities become gradually darker.

And with the older cat, its general body temperture begins to fall somewhat and that causes its body fur to darken as time goes by.