Why does concealer have to be a lighter shade than your foundation shade?

[Vixxan]

I never use concealer lighter than my skin tone. To make sure I have the perfect shade of concealer I mix a little of my foundation with it.

 

[elektra513]

This is for undereye areas only, but I feel that the concealer shouldn't lighten the area, it should brighten the area, which is different. Specifically with darker undereye areas, lighter concealers can make them look worse. Two shades lighter than your skin tone? Horrendous IMO. Using a salmon corrector like someone mentioned earlier can cancel out the darker spots so the skin tone there can "go" with the rest of the face. Then the concealer can be the same skin tone color. I think 1 shade lighter maximum is the way to go, but the tone of the concealer/corrector is much more important than the shade. I actually prefer to mix NC and NW concealers (studio finish in the pot) to make up whatever color I need for under the eye.

For all other spots on the face, concealer should match the skin