Here's why:

* Pigment Production: Serratia marcescens produces a red pigment called prodigiosin. This pigment is responsible for the characteristic pink color of the colonies.

* Growth Conditions: The production of prodigiosin is influenced by factors like temperature and nutrient availability.

* Other Features: Serratia marcescens is a facultative anaerobic bacterium that can grow on a variety of media, including nutrient agar.

Important Note: While Serratia marcescens is a common cause of pink colonies, other bacteria can also produce similar pigments under specific conditions. Therefore, proper identification requires additional tests beyond colony color.