Choose a reason for the colour
A restrained patch of colour can draw attention to one part of an image, but it also changes the balance of the whole frame. Decide what you want the viewer to notice before adding it. An effect is most convincing when it serves an idea.
Keep the treatment coherent
Consider how soft or precise the edges should be and whether the colour belongs to the light in the scene. A gentle tint and a highly saturated accent create very different moods. Compare the result with the untreated version to see what has been gained or lost.
Keep the original alongside the experiment
Work from a copy and save the unaltered photograph separately. Label the edited version so that a creative interpretation is not confused with an untouched record. That distinction is especially useful when a family picture will be passed on to someone else.