Original photo owned by BobNSA and posted in dpr retouching forum

The process is very straightforward, though the devil is in the details.

Basically, you select a color for each section, add it as a new Hue/Sat layer on top of the
black and white. Then mask out all areas that should not have this color. Set the new layer mode
to 'Color'. Also make sure you select 'colorize' in the Hue/Sat option.

Repeat this process for every section you want to add color to.

Finally, now that you have the basic color defined for each area, you can bring up that hue/sat layer and tweak color and opacity anytime you want.

The key is in selecting the right color though. Note, in this example, I also added some shades of pink to the cheek's basic color.

Also, one could work on removing the texture from the image with the usual dust/noise filter and then masking back what one lost.