What color does a positive indole test display when using Kovac's reagent?

A positive indole test displays a red color when using Kovac's reagent. This red color indicates the presence of indole, which is a metabolic byproduct formed by the breakdown of the amino acid tryptophan by certain bacteria. Kovac's reagent reacts with indole to form a red complex, which simplifies the identification of organisms that can metabolize tryptophan. This characteristic is particularly useful in the laboratory for differentiating between various species of Enterobacteriaceae and other bacteria.

In contrast, blue, yellow, and green colors do not indicate a positive reaction with Kovac's reagent. Each of these colors corresponds to different biochemical tests or different results in other contexts but not to the indole test specifically. Therefore, the presence of a red color is a clear and definitive indicator of a positive indole result.