It's very easy, and it is currently legal. Be forewarned that some DPC members do not like it. I think it's okay in certain circumstances, but I advise you not to use it just because it's 'cool.'
In Photoshop, under Image Adustments, select the Hue/Saturation window. Then select each color individually that you want to remove and slide the slider to the desaturate side. Then hope that your colors are pure and that they are where you want them! In this photo, for instance, it works pretty well, but you can still see some red in the man's ear an by his nose. It gets tricky when you've got more things going on.
Your hue/sat window should look something like this...
First of all select a colour in the 'edit' dropdown, then use the coloured bar at the bottom to select the range of colours you want to remove, then move the Saturation slider all the way down.
You may also want to adjust the saturation on the red channel once you have removed the other colours. Just play around with it and see what looks good.
