To save some relationships, please keep in mind you can have mixed twins (one black one white) from the same father. These are fraternal twins from a a multiracial couple. The media (for example) often portrays this as really surprising but the differences in these fraternal (dizygotic) twins should not be that surprising when you think about it. This is because fraternal twins have genetic differences just like you are different from your brothers and sisters. It is just the random chance of receiving different alleles (gene variants) from your parents and these cases the the gene variants that control melanin production the main pigment found in skin.
Love Genetics!
Cheers!
ian