Anyone got any idea why there would be black/dark spots in something I painted?

I made a beige color mixing brown, yellow and white but when I used the paint there's like black stains in the beige color, I added several coats to try removing it which slightly made it better but there's still black stains on it, the only thing I can think of as the issue is that I mixed the paint with a used brush although I feel like I cleaned the brush off well, any ideas on how to fix it without restarting? I was thinking of using a darker tone of the same color and adding a coat of that but I don't know
Anyone got any idea why there would be black/dark spots in something I painted?
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