Dark pimentation on the facial skin is a very common problem. The dark patches could be on any place on the face usually they are seen to be around lips, nose, on the chin area or the forehead. And they could range in sizes. The dark pigmentation has usually been observed in people of colour. I have been asked this question several time of what is the best way to cover these patches, as underneath the foundation and concealers they appear ashy and become more prominent.
These dark patches are the most difficult to cover up as the use of colour correctors or concealers makes them appear more ashy and not very attractive. The best way to cover them up is to use a darker shade of foundation ( darker than your usual) on the patches. If you wish you could apply it to your whole face or only apply it over the patches and blend it into your shade of foundation ( full coverage foundation) really well so that there aren't any streaks or lines. This will even out the skin tone in a very subtle way and the pimentation will be less visible.
Disclaimer : All the pictures in this post have been taken from the internet for the purpose of showing the examples of different looks.