The cats in the film are a group of alley cats who are mostly grey or black in color. The most prominent cat is "Si" (short for "Siamese"), a black cat with a white patch on his chest. However, he is not a Siamese breed.
The confusion may arise from the fact that Siamese cats were a popular breed in the 1950s, and their distinctive markings and blue eyes were often depicted in cartoons and other media. However, they are not represented in the original "Lady and the Tramp".