Colorization is the work of adding colour to a film that was first made in black and white. People take an old movie and put colour onto it, now usually with computers, so faces, clothes, and streets look as if they were filmed in colour. Some film lovers hate this: they say the black-and-white look was a choice, not a mistake. Others like seeing old stories in colour. The word is mainly used about films and old photographs. It names the process, not the person doing it. American spelling is colorization; British English may write colourization. You would not use it for a child colouring a picture in a book — that is just colouring.