Trevor, I believe I've previously mentioned that I have pre-selected my "color of choice" (which is green) from the Fill contents menu (which includes three (3) options: Foregound Color, Background Color, and Color). You guys do use Photoshop/Photoshop Beta right?
Anyway, I have already solved this issue. Somehow within the Fill contents menu, the box for "Preserve Transparency" got checked. Unchecking it solved the problem.

Oh, and by the way Trevor, using Alt+Backspace only fills the open unfilled layer with the foreground color (which is normally Black), provided a "shadow" (for example) hasn't already been filled with Black. If it has already been used, then pressing Alt+Backspace will fill then open unfilled layer with White.