This is by no means a new technique in the beauty world, but it has picked up momentum lately as many companies are coming out with concealers in more than just skin tones. There are green, purple, yellow, etc. Each with its own purpose to help you conceal anything that may be going on.
The concept is to use the opposite color on the color wheel to make the spots disappear. So if you have redness, then use a green concealer to make it vanish.
As mentioned earlier, green is meant to cancel out any redness from acne or scars. If it is widespread from either sunburn, rosacea, or windburn then a green primer would be the best choice as this will help you get the even complexion and cancel out the redness.
This is best used to cancel out any yellow undertones in your skin as it will help bring out a healthy glow. Just like with wide spread redness, any wide spread yellow can be best paired with a purple primer.
As purple is the opposite of yellow, use this shade if you have any blue or purple undertones. These can be commonly found under the eyes to conceal bags. Depending on your skin tone, it can also be used as the perfect eyeshadow base.