Sure, I'll look for some kind of key or legend. On the other hand, though, the colors only tell you whether the forum has new posts or not, essentially, so is it really that important to know?
Well okay there's no legend option I've found so far. So either we make the colors more intuitive or actually put text on there that says "locked" or whatever, which I don't think looks as classy.
I'm also more than willing to upload entirely different buttons if people want to build them. It's just the building the buttons (or finding a set we like) that's time consuming.