There are as many reasons to add color back into black-and-white photographs as there are ways to do it.
You can change the emotional tone of photographs when you add warm or cool tints to them.
You can separate areas of a photograph by toning them differently.
You can enhance the illusion of volume in a photograph by adding different colors into highlights and shadows; typically highlights are warm and shadows are cool.
You can increase volume further by adding gradations of hue in specific regions of a photograph; typically this is done with brushes.
You can add still greater realistic complexity by restoring trace amounts of the original color.
If you increase the saturation of any of these treatments beyond a low level, you turn black-and-white photographs into color photographs.
Read more in my Black & White lessons.
Learn more in my digital photography and digital printing workshops.