My 4 year old daughter says her skin colour is disgusting

When my daughter was in preschool ( literally only 3) a girl kept repeatedly telling her she was brown skinned. Fast forward to reception where it is a predominantly white school, she has been saying her skin is disgusting because she is brown ever since she started. I feel so terrible as a parent and keep telling her she is beautiful, she has lots of family friends of colour too, but nothing seems to be working.