In novels I’ve read, I have seen graphic descriptions such as coffee-coloured skin, olive-coloured skin, pink skin (for a so-called white person). Another approach is to say things along the line of “He was the only person in the room who wasn’t/or who was black.” That establishes the basics. You can also describe the tight curls of a multiracial person and the facial features, explaining the ancestry (and circumstances) that brought about these features. Perhaps your character has strong feelings about this mix of genes. I’m sure you’ll think of many other ways to describe your characters in respectful but vivid ways.