In a previous log I mentioned I wasn't sure what "indexing" meant. Well, I figured it out now. It's a separate layer that is of a single 24-bit color. Well, indexing in general can be any bit depth. But for the purposes of the SmartMatrix SD Shield that value is 24-bit.

What's also interesting is that I then figured out how to configure different layers. I think this will really add some depth and versatility to the display. I envision possibly mixing GIFS and bitmaps on different layers. Or at the least sprites and bitmaps. And with that high color depth it may prove very clear.