Question

stray dogs exhibit a lesser variety of colours than pet dogs

Answer 1

Pet dogs are breaded very carefully and selectively to ensure the unique and quality breed. While the stray dogs belongs to the poor breed and thus do not have that kind of color and fur that a pet do possess. 

Answer 2

Pet dogs are carefully trained and looked after by their owners, they mate them with the best breed. On the other hand there is no one to look after them and they grow on their own they mate with the breed which is available around them which is the reason they exhibit lesser variety.