In-Breeding in rats really isn't a big deal. As long as the line is healthy they can go something like 26+ generations before there'd be any sort of weirdness cropping up.

As for Siamese rats they are a beige rat with brown color points. The color points are affected by temperature, the colder the rat the stronger the points. With a self the rat should be all over creme colored with dark brown on their feet, nose, base of the ears and base of their tail fading up the back. The color points only show up where the beige is. So if the rat has a hooded marking it will only have the color fade up the hood with white sides. Or if the rat has a blaze, it won't have the color point on its nose.

Some times if you breed a Siamese to other colors, say a Siamese to a blue, the blue babies may still get color points, they'll just be darker blue points. Generally this is frowned upon, a self rat shouldn't have color fluctuations. Typically breeders won't breed for a marked Siamese line since it tends to mess up the Siamese coloring/markings.

Himalayan and Burmese are other colors that are color points, the Himi's being white rats with brown points and the Burmese being light brown with darker brown points.

Anyways if you're breeding feeders it doesn't really matter, if you are breeding to breed Siamese rats, mix them with black selfs to make the color stronger.