Hand-coloring has been used by photographers for over a century to add color to black and white photographs. This was done by hand painting the photograph with oil or watercolor paints. Unlike toning, which applies the same color to the entire image, selective coloring applies one or more colors to specific parts of the image.
Does this sound familiar? You have a photograph, but the subject of interest is not where you want it. You want to want to place the subject according to the rule of thirds. You try to crop off one side of the photo to put the subject at one of the intersection points. So you try to eye-ball your crop, but it doesn't quite work. You undo and try again. And again. And again. You start using rulers and guidelines. You count pixels. Nothing's working. Out of frustration you throw your computer out the window and set fire to your house. We've all been there.
Here's a fantastic tutorial on how to crop to the rule of thirds without losing your marbles.