I've been playing about with the Image::BioChrome module by Simon Matthews, and it inspired me to do something very similar to colourise greyscale GIF images. BioChrome gives you extended control over the way in which images are colourised, but at the cost of requiring a specially prepared source image to work from. BioTint instead allows you to feed it a regular greyscale image and will colour it in a single colour.
The borders around the images below have been coloured using BioTint. The left slide shows the original greyscale images, with the ones on the rightshowing coloured versions.
BioTint makes it quick and easy to get passable results but it's not perfect. Notice how the drop shadows in the above examples are also coloured when they should really be neutral grayscale. However, you can easily do that using BioChrome by using the red channel for the colour channel and green for the drop shadow channel.