To turn only one layer black and white you can use a couple of different adjustment layers, making sure to 'clip' the adjustment to the layer you wish to affect.
HSL (Hue, Saturation, Luminosity adjustment) would work, by setting the Saturation at -100% on the Master channel. There is also a Black and White adjustment layer. It automatically turns the image black and white, and allows you to adjust luminosity of the different colors involved. You could keep red very light gray and green darker grey, for example.
On most, but not quite all, of the adjustment layers you can 'clip' the adjustment to the layer directly beneath the adjustment by clicking on the small icon at the bottom of the properties of the adjustment that looks like a square with an arrow coming out on the left. Or, use the menu command Layer > Create Clipping Mask.