The two I have are around the 4-5" mark, maybe a little bigger, but their dorsals aren't as long as yours 'cos they keep scrappin'!
They tend to change from pale brown to almost black, depending on the light conditions and their mood. Seems to be the more relaxed they are, the darker they get...
Does yours like to hang out upside down a lot when he's resting under rocks and driftwood..?
My ones do, hence why they get confused with true upside down catfish. They are often sold as such at lfs.