Basically any color on the wheel cancels out what is across from it, for example a green concealer would cancel out any redness.
Purple or violet concealers get rid of any yellow undertones that your skin may have, and evening it out in the process.
The color green cancels out any redness you may have under your eyes or caused by blemishes.
This collection of pink/salmon/peach concealers work for brightening the face and work great for under eye circles.