I want to colorize the white part of an image

[anthony]

there is no simple way for IM or any other image processor to separate the parts your want to color and the parts you don't.

The best idea is to generate a mask, either use some advanced image segmentation method (which IM has not gone into yet) or use an editor to generate a mask of the area you want colored.

IM can then color images using that mask to what ever color you want. You can even '-compose Overlay' images for different effects too, though that works better with a grey starting image.