1. Duplicate your background layer.
2. Desaturate that layer (i.e. make it b&w).
3. Apply a layer mask to the layer.
4. Use the black brush to paint on the layer mask (make sure that is
where you are, not on the image itself). That will bring the colour back
to those areas. If you make a mistake, change the brush to white and go
over it. Using a masking technique is best because you can keep
adjusting it and fixing mistakes, which can't be done easily with other
methods such as the eraser.
5. When you're happy, flatten and save.