Do you love black and white pics? But sometimes you need a little color pop out? OK, let's start.
Open your pic and duplicate the layer. Now go to your layer palette, press your Alt-key and click on the little black/white circle and go to Hue/Saturation:
Check the box where it says group layers and press OK.
Now we created a new adjustment layer and a new window is popping up. Put your saturation completely to the left and if you want, you can play a little with the different colors.
Go to your brush now and paint on your adjustment layer. Black brings your original color, white takes it back to b/w.
When you're finished and like what you did, just merge layers and you're done!
Hope you liked this little tutorial.