I knew a man, many years ago, who was the inspiration for a story I wrote, who had darkish skin (most likely a combination of smoking around a packet or two of cigarettes a day, and his work as a truck driver. If he sat shirtless, he had lighter skin on his chest) yet his sister was quite fair. He believed in white supremacy (he'd allow black or Asian people near him if it suited him) and after his mother died, his sister, who was not racist, told him that their great-grandfather was Chinese. He lived in denial about this.
White-passing is white supremacy because what a person who says it is really saying is that they erase the person's ancestry and they don't see the person for who they are.
I know, I feel guilty, because my great-great-great-grandfather was a slave trader who married a West Indian woman, in St Vincent's and the Grenadines, where he lived, when I go to the shops as sometimes a lovely young woman, who has black hair and really dark olive skin, will ask me if I want her to pack my bags and I am reminded of how my ancestors viewed POC, and want the woman to feel equal. I don't want the woman to think I notice, though.
I did come to the defence of a person who was accused of being racist because they said they were an eighth Aboriginal, and I replied, "How he chooses to identify is his business. He's not being racist, he is just acknowledging that one of his great-grandparents was Aboriginal."
I deplore racism and white supremacy, and whenever I tell someone that I'm part West Indian and they say, "Oh, but you look Anglo-Saxon," I reply, "I have brown eyes and brown hair, which come from my Irish side, and my West Indian ancestry is verifiable." I don't tell people I'm West Indian, though, as I am still learning about the culture of St Vincent's and the Grenadines.