Black is black to me. I'm not really a fan of sliming them up too much. Add to much slime and then it starts looking like one big convulsing mess. I was sort of okay with that brown-ish coloration in A:R, but only sort of. I just wasn't really feeling it at all.
I'm still not opposed to silver Aliens like the very old AVP rumor mill once hinted at, but I'd love to see a good story-based reason as to why they would be.