Skin color is not a good metric for judging black or white. There's a lot more to it, skull shape and size is a better metric. Nose too.
Dell Curry is brown/black skin color, but his nose is long and straight, something not present in blacks naturally. His skull shape is narrow and elongated which contrasts with wide and short black skull. He has no facial prognathism which is also uncommon for blacks.
In short his skull shape, nose and facial protudence is mostly white. He looks quite similar to some East Africans who are a historically mixed people of semittes, caucasians and nubians (blacks).
As for Steph he is almost entirely caucasoid in his appearance except his hair and skin color, he even has light eyes, one of the best markers that a person is mostly white.
Look at Stephs nose that is a 100% white caucasoid nose. Look at his high forehead which does not slope.
I wouldn't be surprised if Step is something like 75-80% white.