Intro: Color Screening Digital Pictures
Galmour shots charges $75 to do this black and white with a color emphasis thing. Well, their lab does the work, and they use photoshop (amazing the information you can get if you're friendly and play dumb). They also showed me their technique and a few other useful tricks....
Step 1: Grab and Image and Make a New Layer
I never work on the original layer, I always use a copy - you can always turn off the base layer.
Make any adjustments to this layer... adjust curves, light levels etc. This is a good time to clean up skin blemishes etc. (I did not do so for this picture, this was just a quick write up as an example).
Step 2: Create a New Layer Based of the Adjusted On
Place this layer on top and desaturate (remove all color or colour if your prefer :P).
Now, using the eraser tool, remove bits of the top layer (the B&W one) to reveal the color(s) below. Make small strokes and go slowly, you can always use the undo feature of your photo editor to fix mistakes.
Step 3: End Result
Now you can resize, save in additional formats etc.
I always do these things at the end of a session so I can utilize the history brush should I need to.
Other things to highlight:
Lips, eyes, hair, jewlery, props, etc.
But be warned, too much is a VERY BAD thing. And sometimes it just looks evil or puke awful. Simple is best and use any bit of artistic judgement or even get a second opinion :P