Open the color image you wish to convert. Create an adjustment layer by choosing "Layer" then ?New Adjustment Layer." At the bottom of the adjustment layer window, set the "Blend Mode" of this adjustment layer to "Color."
Create a second adjustment layer by choosing "Layer" then "New Adjustment Layer." Slide the Saturation slider all the way to the left.
Now that you have two adjustment layers, select the first adjustment layer you created and double-click the small icon to reopen the Hue/Saturation window for that adjustment layer. Slide the Hue slider to the left to -150 or so.
Click "Layer," then "Flatten Image."
