How to select the foreground color in a graphic

How can I edit a graphic to select the foreground color so it will be
whatever WinXP is using? I have a graphic (.GIF) with black text, and I
can select a transparent background in MS Paint. However, when I use an
accessibility scheme with a dark background, the black graphic text is
unreadable. How can I specify a color in a graphic so it matches the
foreground text color, or alternatively, how can I detect the dark
beckground and select an alternate graphic with light colored text?

- Robert