What color indicates a non-lactose fermenter in MacConkey Agar?

In MacConkey Agar, a colorless appearance indicates a non-lactose fermenter. This specific medium is designed to differentiate between lactose fermenters and non-fermenters based on color change due to pH shifts. When lactose is fermented, acid is produced, which lowers the pH and leads to the formation of a reddish-pink color in the surrounding medium due to the pH indicator in the agar. Non-lactose fermenters do not produce acid from lactose fermentation and thus do not cause this color change, resulting in a colorless appearance of their colonies on the agar.

The other options represent various reactions based on the presence of lactose fermenters or different indicators in the medium. For instance, reddish-pink indicates a positive reaction for lactose fermentation, while brown and dark purple do not relate directly to non-lactose fermenters on MacConkey Agar.