First you take a coloured image like this one:
Next you turn it black and white. I use the Black and white tool in CS4, however you can use Channel Mixer or just desaturate it. Channel mixer tends to give you more control over the final result. Do not use Greyscale as it won't work!!!
The Channel Mixer and Desaturate are in Image-Adjustments on top tool bar.
Once you reach this stage go to your tool bar on the left hand side and select the History Brush tool it's the one with the brush symbol with a arrow curving over the top If you click and hold it will give you the choice of two brushes make sure you select the right one. NOT the Art History Brush!!
Ok now the arty bit. Zoom in to the item or items you want to paint back in and start painting colour back in. For this image it was the three red cars in the centre of the bridge. Change your brush size as you go, for more detailed areas. It takes practice and a graphics tablet is a real help, but with patience you can do it without a tablet.
And there you have it a rather cool effect and easy to do. If you have any problems let me know.