What color change indicates a presence of starch after adding iodine?

The presence of starch in a sample is indicated by a dark blue or purple color when iodine is added. Iodine forms a complex with the amylose component of starch, resulting in this characteristic color change. This reaction is a classic test for starch, and it is used widely in microbiology and biochemistry to confirm its presence in various substances.

While the other color options may suggest different biochemical reactions or the absence of starch, they do not indicate a positive result for starch presence. The specific formation of a dark blue or purple color upon interaction with iodine is what distinctly marks the presence of starch in the solution.