How does stain fighting & color boosting bleach work?

These detergents use what are called “optical enhancers” or optical brighteners.

But how do the “enhancers” know between a color to be enhanced versus a stain to be zapped out?

The enhancers don’t necessarily remove the stain. The detergent does that. Detergent doesn’t typically remove color from clothing (although, this can happen with newer clothes made with certain dyes, of course.) The detergent works to break down and remove the soil from the fabric, while the optical enhancers create a chemical reaction on the fabric that makes the color appear more vibrant to the human eye by altering the way that light is absorbed and released. There are two separate things at work.