I've heard various reasons. No black guys could mean she doesn't want pimps trying to recruit her, possibly because she has a black pimp of her own. It could also be that blacks have big dicks and stamina, so she gets tired or sore from fucking too long. It could also mean she's a scammer and blacks are less likely to put up with her bullshit. If she's a FOB Asian, she may have never encountered blacks in real life and is scared of them due to the various stereotypes.
No brown guys is usually because brown guys, especially recent immigrants, tend to haggle or have bad hygiene. They can also have disrespectful attitudes towards women in general. I've also heard brown guys are more likely to push for BBFS or even try stealthing.
Is this racist? I say yes. Having a negative attitude toward someone solely because of their race is the very definition of racism. Is it acceptable? I also say yes. Her body her choice. This is one area in life where racism is understandable. Her choice is still racist though, despite all the crybabies who will try to claim otherwise.