Now that summer is here (at least it is here in Florida) issues with skin tonality – sun burn, tan lines, etc – will become an issue.
The technique I use to address this is to use a layer, set to hue (occasionally color) blend mode, and filled with a color sampled from a nearby skin area. Then, using a layer mask, I reveal areas I want toned (the discolored areas of the skin) and use opacity to finalize the look.
This can also be used for hair color – in some of the images her hair was too light for my taste so I retuched sections using a color sampled from another area of the hair.
Before and after images: