Basically any non black or transparent pixel should be set to white.
To be clear, is this really what you wanted? If so, my avatar…
… would look like this…
Which is kind of a cool effect, but did you instead want to make it grayscale (which is what a “black and white” movie is more like)? So it looks like this instead…
The algorithm changes a little, but both are simple bits of processing on an SDL_Surface.