People come in all shades no matter the race people. White people vary in skin tone, personally I'm quite pale, while my friend is very tanned (not fake hehe) and that's in England and as u know we have like a month of sunshine max.
ASnyway its the same with other races too, there are plenty of asian and black people with varying tones of skin, some lighter than others.
However in rhianna's case, she's also lighter than the others because she's mixed race, her dad's irish.