This is the image we will be working with:
First, we want to make a selection around the area we are wanting to change the color of. It does not need to be perfect, but we need to isolate the area particularly from other parts of the image that may be similar colors. We make a selection like so:
Now we goto Image > Adjustments > Replace Color:
We can use the eyedropper tool to select one of the shades of red from the lips. Then we can adjust the fuzziness to expand the selection of colors. Notice that when we select a color, the preview window shows us the area of the image that will be replaced:
Once we have adjusted the fuzziness to the desired level (when enough of the lips show to color properly), as below, we can adjust the Hue, Saturation and Lightness sliders:
As you move the sliders, you'll see the lips change color nicely. This technique can also work well with changing eye color. Some variations are below:
More Tutorials: 
