What color change indicates the presence of starch when using iodine solution?

The presence of starch is indicated by a blue-black color change when iodine solution is applied. This reaction occurs because the iodine molecules fit into the helical structure of starch, forming a complex that produces this distinct coloration. Iodine in its natural state is brown, but upon interacting with starch, the iodine's color shifts to blue-black, making it a reliable test for starch detection in various samples, such as food items or biological materials. Thus, seeing a blue-black color signifies that starch is present in the tested sample.