I think you could use any
color you want. Most people use cream or offwhite because this is an aran
afghan, and arans are traditionally those colors. Here's
one knitted in several different colors.
Personally, I wouldn't use anything that can't be machine washed. But that's just me. If it's going to be used as a blanket, it really needs to be washed. But there are many lovely washable yarns out there. I used Plymouth Encore. It's machine washable and dryable and very very soft and cozy. But honestly.. look at that afghan. It could be knit with kitchen twine and still be gorgeous. LOL
Anyway. if you want to use an Alpaca, there are several different colors to chose from. Alpacas are naturally white, cream, grey, tan, rust, camel, brown or black.