1. Open the image.
2. In the Layers palette, click on the Hue/Saturation adjustment layer icon (black and white cookie tool).
3. In the Adjustments tab, click on the plus eyedropper and drag over the object you want to keep coloured.
4. Once you’ve determined the colour, click the drop down box, select every other colour, and desaturate.
5. In the Layers palette, click on the Hue/Saturation adjustment layer icon to create another adjustment layer.
6. Desaturate Master from the drop down box.
7. In the Toolbox, select the Paintbrush. Adjust the size to the object you want to colour.
8. Reset the default palette colours by clicking D on the keyboard.
9. Make sure black is set to the foreground colour and paint over the object you want to colour.
For printable instructions, go to http://www.scribd.com/doc/87289465