Hitokage said:
Select by Color
Invert Selection
Desaturate
That'll only work for images where the colour borders are very clearly defined. You're much better off with:
Image > Adjustments > Hue/Saturation...
Just reduce the saturation on everything bar the Reds (and the Master) to 0, then move the boundaries on the Yellows and the Magentas (to encroach in on yellowey reds and magentaey reds) until you've only got the range of reds you want to keep. (You can also increase the saturation on the Reds if you want them to stand out a bit more, although this obviously depends on the image you're working with and what you're trying to achieve)
Just a note, though, the image in the first post was probably a simple Polygon-Lasso, Invert and Desaturate job, as the natural skin tones would have contained quite a bit of red in the first place.
