In the first scene where Tramp is introduced, he is seen sleeping. When he wakes up and comes out, his fur is the color brown. Even after he bathes, he is brown. Throughout the rest of the movie, he is a gray color. This may be because as the progression of the movie went on, they thought gray was a better color, or more suitable. I noticed this when I was 6 years old, and have been watching ever since.