I'm running a google search and finding a lot of interesting quotes, but I really thought there was one that said directly that black people would become white in the paradise. Maybe I'm just misremembering from these quotes, which still make it pretty clear:
"A little while, and the Millennial kingdom will be inaugurated, which will bring restitution to all mankind--restitution to the perfection of mind and body, feature and color, to the grand original standard, which God declared "very good," and which was lost for a time through sin, but which is soon to be restored by the powerful kingdom of the Messiah (Watchtower, April 1, 1914:110).
No. But... what the Ethiopian cannot do for himself God could readily do for him. The difference between the races of men... have long been arguments against the solidarity of the human family. The doctrine of restitution has also raised the question. How could all men be brought to perfection and which color of skin was the original? The answer is now provided. God can change the Ethiopian's skin in his own due time... Julius Jackson, of New Frankfort, Montana, a negro boy of nine years, began to grow white in September, 1901, and is now fully nine-tenths white. He assures us that this is no whitish skin disease; but that the new white skin is as healthy as that of any white boy, and that the changed boy has never been sick and never has taken medicines (Zion's Watch Tower, February 15, 1904: 52-53).