I have some off-white or natural or cream colored alpaca (am not sure what color you'd classify it, I found it in a thrift store). Anyhoo, I felted a test swatch of it, and it felted wonderfully. In fact, I keep the little test swatch here at my desk and find myself stroking it throughout the day. It seems to relieve stress, kind of like one of those worry stones. :lol:
As someone here stated, it's probably the bleach they use to "whiten" yarn which causes it to be unfeltable.