there is so many technics to do that, you may try desaturated selective color in hue/ sat, or duplicate layer in b and w and remove all the colos with eraser tool.. something like that i think
If it is a small area that needs desat I copy the layer and use the "sponge" tool. The sponge tool is part of the 3 tools grouped with the burn and dodge tool.
It is probably the most least used technique but for small areas it is the best! (this technique NOT legal in basic editing) !
Well the easiest way to accomplish that would be to go into the adjustments/saturation and desaturate every color besides blue and cyan. Then take the desaturate sponge (with a big brush size) and just desaturate everything except the eyes (you'll want a smaller brush size when you work around the eyes).
