Never mind. Figured it out!

Paint me pink with anticipation. What does it mean?

There is also a gray background, and on the right a single gray character. Do all of these things mean something?

In my experience, the pink background indicates a problem with either the glyph name or its Unicode number. For any given character set, the gray character means that the glyph slot is "active" but there's no glyph in it, and the gray background means the glyph belongs to that character set, but there is no glyph present.

If you have Python installed and go to Macros and "Mark Problems" it shows up pink when points are not at the right place or when point should be inserted.
@oldnick. Unicode problems show up in red, but only at the unicode value, not at the part were the glyph is previewed.
Grey means that the glyph is part of the characterset, but is not active. If you would make an empty glyph, it would not show up grey.
...on the other hand.. I use FontLab on Windows (that's an OS made by a company called Microsoft http://en.wikipedia.org/wiki/Microsoft) so it could be different on a Mac

I agree with Micha. When I've run into that it's usually something along the lines of a stray node or unconnected outline.

This coloration can also be manually invoked by the user, just as a way of tagging a glyph for future reference. (Which could happen by accident.)

Pablo, instead of repeatedly posting "never mind, I figured it out," perhaps you could share your discoveries?