What color indicates a positive reaction in the iodine test?

In the iodine test, a positive reaction is indicated by the development of a dark blue color. This test is primarily used to detect the presence of starch. When iodine (typically in the form of potassium iodide solution) comes in contact with starch, it forms a complex that gives off a deep blue or black color. This color change occurs because the iodine molecules fit inside the helical structure of the amylose component of starch, resulting in a characteristic coloration.

The other options do not represent a positive reaction for starch. Yellow typically reflects no significant interaction with iodine, while green and no change indicate the absence of starch. Therefore, the presence of a dark blue color confirms that starch is present in the tested sample.