Iodine test for starch. Test tubes showing colours for positive results of the test for starch using iodine. The colour varies with the concentration of the starch. From left-right the concentrations are: 1%, 0.1% and 0.01%, decreasing 10 times for each test tube. Starch is a carbohydrate molecule found in food like bread. Its molecule is long and coiled, and the iodine ends up in the coil, which is why the iodine turns from light brown to purple and appears black at high concentrations.