What IS a natural alpaca color? I am imagining a light camel, which would be PREEEETY, but I am probably wrong.
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. 
Ooooh i love the multi-color! I am going to work on a pillow similar (but much simpler) to that! Basically a “build your skills” pillow! I was going to go all one color… but now I think maybe I will do a few. 