What is Keylight? Keylight is The Foundry's powerful chroma-keying plug-in that comes free with the Production Bundle versions of After Effects 6. It won an Academy Award for Technical Excellence, and it's a one-stop shop for keying, spill and color correction.
When you apply Keylight to a layer and choose a color to key out, two things happen: Keylight erases all the pixels that match the key color and it removes traces of that color (spill) from the other pixels. So if you key out green, all green (or near green) pixels will turn invisible, and the rest of the pixels will have their greenness reduced.