But maybe you don't have the lip stick or lip gloss you want.
Never mind, PHOTOSHOP can help you solve this problem! ^^
Open and choose the photo that you want to edit the lips colour, right click the layer and click Duplicate layer.
Then select the quick mask tool.
click on the brush tool.
Now colour the part that you want to retouch. The mask will create a selection.
Click on the quick mask tool to create the selection.
click on the Rectangular Marquee Tool.
Right click on it and select inverse.
Right click again and select feather.
1 pixel is enough.
Then click on Image> Adjustment> Hue/ Saturation.
Adjust the colour that you want.
Use Rectangular Marquee Tool and click on the lips to dis-select the selection.
Lastly, use the blur tool the blur the lips outline to make it look more natural.
Here is the Before and After.