I've been playing (should be working) and stumbled across this today - I'm sure I've heard it before, but not in these words.
Take a black and white image, fix the contrast etc, crop it and then save it if you don't trust yourself
Now add a Levels Adjustment layer - don't make any adjustments, just click OK. Fill the mask area of the Levels Adjustment layer with black, now hold down the Alt (or Option) + Shift and click on the mask to activate the red cover on the image (this makes it easier to see where you're painting) Now switch to white paint and a hard brush and start scraping the red away from some area of your image. In the image posted below, I did the boys shirt.
Do the white/black switcheroo until you have a nice clean selection. Now you can do Alt + Shift and click on the mask again to get rid of the mask.
Now double click on the Levels icon in the Levels Adjustment layer so the dialog box opens. Pick Red from the channels drop down menu and put 51 in the right hand box at the bottom (where it says output value), do the same with Green - value = 102 and Blue - value = 204. Click OK and voila, the shirt has changed to a nice blue. You can further affect the color by changing the blending mode of the levels layer to Color.
I did the same with the skin using the following values - Red=254, Green=228, Blue=201
I got the RGB values from the color picker. I've been playing around with this most of the afternoon trying to find a simple way to do it and this just happened - if it's been presented before, I'm sorry but I missed it - or I read it carefully and stored it in some inaccessible part of my brain that feeds me other peoples ideas as if they were my own
Ok, here's the pic, Take care, Margaret
My selection on this example isn't very good and I haven't even tried to make the skin realistic - that wasn't my intention for this demo.