How can no color change in the broth and a conversion to a yellow color both be considered negative results?

Explanation:

A Pure bacterial culture must be grown in sterile tryptophan or peptone broth for 24–48 hours before performing the test. A sterile environment helps to avoid contamination.

When a test is conducted and it shows the presence of a red or red-violet color in the surface alcohol layer it means the result is positive

But when the colour is yellow it indicate negative reaction due to glucose fermentation.

A deviation from two color may also occur when we have an orange color. This colour change is due to the presence of methyl indole

When there is no color change this indicate No decarboxylase which is an enzyme needed for the fermentation reaction.

Color changes are indicator of how successful the reaction was.