I'm not sure how to colorize film, but you can make it look very good if you have accurate refference material. The problem with all 3 of the colorizations of It's A wonderful Life is that they didn't have color photos of the sets and people. The main problem with any colorization is that the colorized footage lacks varied colors. For example, a person's skin is made not only of different shades, but of many different colors. If anyone wants to take the time, it is possible to do it frame by frame with photoshop. Right now I'm going through ANH:R for accurate frames to compare for color-correction of the deleted scenes.