Branwen's Feather is described as a blackened purple (yes, another one...shocker!) reminiscent of a raven's feather. Yeah, that pretty well describes is all right. Branwen's Feather has a deep black base with purple and multicolor shimmers. Gorgeously vampy is how I describe it.
Again, apologies for the craptastic photos. Just picture this color richer and not so washed out and you'll get an idea of how awesome it is.