Various dyes, pH indicators, and other ingredients are included in differential plating media to serve as indicators of enzymatic activities and aid in identifying bacterial isolates.
For example in MacConkey agar, lactose fermenting colonies appear pink and lactose nonfermenting colonies appear pale or clear whereas the lactose fermenting colonies appear yellow in Hektoen-Enteric agar. This is because of the production of acids and subsequent changes in pH. As different pH indicators are used in these media, the colors of the colonies are different.
