Some of us a plagued with really dark circles, while others get off easy with minor circles. The following color theory will teach you how to drastically reduce their impact:

Let’s work on the assumption that most dark circles are blue in undertone.
If you are fair skinned, you can reduce circles with yellow concealer. Medium skin can use an orange/salmon concealer and dark skin can conceal with a violet or even dark orange/coral.

After neutralizing your circles, use a concealer that is about 1-2 shades lighter than your skin tone. Using a light concealer will brighten up your under eye- this is key.
You should not conceal dark with dark because that will accentuate your circles.

What are your little tricks to hide circles?