1. Make a copy of your original layer then go up to adjustments and use black and white.
(If that's not dark enough for you, then go up to adjustments and use brightness/contrast to darken it up. Be sure you don't make it so dark you can't see the details or it will look bad.)
EXAMPLE
2. Put the original layer on top, in this case I'm turning goku's undershirt, boots, and wristbands black. And use the wand or the eraser to delete all the color you are replacing.
EXAMPLE
3. Now flatten the layers and you're done.
