If you inoculated a lactose phenol red broth with a bacterium and after 24 hours, the broth turns yellow. You accidentally leave it in the incubator an additional 48 hours, after which the same broth turns into a dark red/pink (magenta). This process is called

 Deamination

Explanation: Deamination is simply the removal of an amino group from a compound, in this instance, from peptone in the phenol red broth.

Within 24-48 hours is the time frame which bacteria can be able to fully ferment carbohydrates (glucose or lactose) and after 48 hours, carbohydrate runs out because it has all been utilized leading to a rise in pH (alkaline) as the bacteria utilize peptones to give alkaline by-products.

substitution is the answer in this case