Of course, you can always make a color photograph black & white by “desaturating” it (Image >> Adjust >> Desaturate) however, this technique leaves the original photo quite blah. (See bottom of tutorial for comparison/contrast study).

I begin with an image:

Next, I create an Layer >> New Adjustment Layer >> Channel Mixer and select Monochrome. I bring the red down to 0% and the green to 100% (although sometimes a better or more realistic look can be achieved by setting blue at 100% and green at 0%; it’s good to try out both settings before going with one or the other:

And since this is an adjustment layer, you can always choose to hide it and bring back the color.

Now compare the left (channel mixer) to the right (desaturate). The dfference is subtle, yet distinctive.