Another way this can be used is to reduce the saturation of all the other colors just a little. This can make one color seem to really pop. Works well with reds e.g. lipstick, roses. It took me ages to figure out how this was done in some TVCs as the reds looked too red to be within gamut.
Grazie, thanks for posting your tutorial...you've taken what is common knowledge to some and made it accessible to the rest of us in a very straight forward manner. Our dance company did a performance of "Beauty & the Beast" with a nod to Cocteau and I have been wanting to produce the performance video of it in B&W with only the reds and this has made it possible to do...Thanks!
My approach allows for multiple layering of a colour and have them come in and out of view on a whim. Once getting a hang of there being a controllable Alpha Channel for a colour then there are other options too.