HMMM. While it isn't specificly mentioned I have some oh-so-soft alpaca I just finished spinning in very dark brown. It would be much nicer than pure wool, even merino. I guess the reason for wool is fire hazard reasons, and alpaca would certainly qualify for this. But sometimes these organizations can be so opinionated, and reject perfectly good items that should be just as good - and in this case superior -to what they recommend.
I also have some nice brown heathered (winds up being med-dark) natural brown colored lamb's wool with about 10% angora in it. Also very very soft, and would not be brightly colored at all. I don't think they'd notice the angora unless I told them it was there.
I spin these drab colors because my brother the truck driver won't wear anything else. But he secretly enjoys a touch of luxury when driving up in New Hamshire or Maine this time of year. And he's had big burly dock workers ask where he got his warm ski band.