Updated Feb 2012.
First, you need to open your image and then duplicate it in the layers panel by right-clicking and selecting ‘duplicate layer’.
Once you have your duplicate layer, go over to the toolbox and select the Zoom tool. This will let you zoom into the area you’re working on and make it easier for you to apply changes to the image.
Next, go to the toolbox and select the Free Select tool. Towards the end of the toolbox you’ll see a series of squares under the free select title. Make sure the second one along (add to current selection) is selected. From here, click and draw around the lips until you get a line of ‘running ants’ surrounding them.
Go to: Colours>Hue/Saturation to open the Hue/Saturation window. In here you can adjust the hue, saturation and lightness until you find a colour you’re happy with. Click OK to apply your changes.
Find the Select menu and click 'none'. This will remove any selections you have made. Finally, zoom back out using the Zoom tool to see the finished result.