Weird color change

When I got my kitten at 3 months old, his tail was black, the same color as the markings on his face, ears, and 2 "other parts." He is now 7 months old, and his tail is a very light tan color. He used to have some black spots on his back legs that have turned tan as well. He also has a lot more tan on his whole body. He used to be mostly white with some tan splotches. I was looking at some old pictures of him, and I can really see the difference. Is this weird? I know that cats can change color slightly as they get older, but he is still a kitten, and this has happened in just a few months. I'm not sure if the breed makes a difference. He is described as a Siamese mix on his adoption paperwork, but a seal point DSH on his rabies certificate.

No, not weird.  It's nut unusual for a cat's colours to change between kittenhood and adulthood.  My Abby was light tan and white when she was a kitten. Now she is orange and white.

Not to worry! Since you have a Siamese mix kitten, then expect the coat color to change even up to two years. I rescued my Pipsqueak when he was a kitten and he was pure white with blue eyes. By the time he was three, he was in full color and a tabby flame point. Enjoy the beautiful color changes that await you! Of course, there are some health concerns with an older cat and coat color or pigment change but that is not the case with your kitten.