I'm not sue if it's racist or not TBH I understand the whole "they're Zombies doesn't matter what colour they are" school of thought which is in a way true , Zombies are Zombies regardless of colour.
But in our modern world context I think it may be an insensitive view of things , if people are offended by this then we should question why and possibly atempt to resolve the situation even if the offence apears to be illogical to you.