You know they exist, but the rest of the world doesn't! Here's a useful trick to cover dark circles under your eyes: use a red lipstick before applying foundation or concealer. Yes, you have read it correctly: Red Lipstick!
How does it work? Simple. Dark circles are two toned: blue and brown. The exact opposite of these two colors are red and orange. When you use a bright color above the dark one, the tone neutralizes. Voila! Your red lipstick is your wand, do your magic!