What color does litmus turn in alkaline solutions?

Litmus is a dye that reacts to changes in pH levels, and in alkaline solutions, it changes color to blue. This color change occurs because alkaline solutions have a pH greater than 7, which increases the concentration of hydroxide ions (OH⁻) in the solution. The chemical structure of litmus alters under these conditions, leading to the blue coloration.

In contrast, litmus turns red in acidic solutions where the pH is less than 7, signifying the presence of more hydrogen ions (H⁺). Yellow and green are not associated with the litmus test; these colors do not represent the behavior of litmus in the context of acidity or alkalinity. Thus, blue is the color indicative of an alkaline environment, aligning with the correct choice.