It is taken for granted that most anime characters (unless otherwise stated) are Japanese. As far as why they have big, round eyes, the answer is that most anime as we know it is influenced by the work of one man. Osamu Tezuka who is known as the "father of anime," kind of the Walt Disney of anime in a way. And, actually, as a kid Tezuka was inspired by Disney's early works like Steamboat Willie and Fleischer's Betty Boop. So, when he began drawing, to him the big round faces and round eyes were just a characteristic of cartoon characters in general, those things were not meant to be a comment on ethnicity. They still are not. As far as hair/eye color. Well, a lot of Japanese people have brown hair rather than black, and lots lighten their hair. Otherwise, I think that it's just done to make things interesting.