Because I haven't until just now. Apparently it's a little brighter than off-white, and, when you take into account average cape sizes and the way they fit on the player models, it looks a little more elongated than most.
Yes, I know I'm way behind on this one, but I think this just overtook Leeroy as my favorite WoW meme. I can't stop laughing at the "visual aids."
Related: Warlocks ride in a file number that is not greater or less than one. (Thanks, druid dude!)