Image/Pixbuf: How to set colour "behind" (semi)-transparent data?

[Toralf Lund <toralf procaptura com>]

When drawing a pixbuf which has an alpha channel, how do I set the 
background colour? I'm talking about the colour that will "shine 
through" fully or partially where alpha value<255. (Seems like a grayish 
colour is the default; I actually want black instead.)

- Toralf