Why is there a leading "#FF" in hexadecimal color values?

That may be the alpha component of the color, which represents the opacity (00 -> transparent, FF -> opaque).

MSDN seems to agree with this: Color

16-bit hexadecimal, alpha -- #AARRGGBB

I believe the leading FF is the alpha. 255 (or FF) being 100% opaque, and 00 would be transparent.