Aisha and Nex look like a black and white couple. I say “look like” because they’re not from Earth, so they don’t identify themselves as those races. But calling them “black” and “white” may serve another purpose: a visual pun.
Wikipedia defines “visual pun” as “a pun involving an image or images (in addition to or instead of language), often based on a rebus.” (A “rebus”, according to Oxford Dictionaries, is “a puzzle in which words are represented by combinations of pictures and individual letters.”) How do Aisha and Nex’s skin tones form a visual pun?
It’s part of their theme: yin and yang. Aisha is “black” and Nex is “white”, and the halves of a yin-yang symbol are literally black and white. So Rainbow may have made Nex white, the opposite of Aisha’s skin tone, as another hint to their yin-yang theme.