The brown from what I've been told is actually Onxy that's faded over time. Wish the dark Onxy was darker, it's not really black just a dark brown. If anyone with want to correct me...
After further review, I'm going to say that I'm 99% sure that the "brown" really is just faded Onyx.
I was looking at my collection of colored SAKs last night and was surprised that I couldn't find my Onyx among the others. After searching for a while (there's only 32 knives in this display case... can't be that hard, can it?!?), I discovered that I was seeing a light brown and a dark brown. Hmm... I think I'll start storing that display upside down to avoid light and future fading.