Why does it turn to a blue/purple color when u mix iodine with starch?

Here is an explanation I pasted from Wiki:

When starch is mixed with iodine in water, an intensely colored starch/iodine complex is formed. Many of the details of the reaction are still unknown. But it seems that the iodine (in the form of I5- ions) gets stuck in the coils of beta amylose molecules (beta amylose is a soluble starch). The starch forces the iodine atoms into a linear arrangement in the central groove of the amylose coil. There is some transfer of charge between the starch and the iodine. That changes the way electrons are confined, and so, changes spacing of the energy levels. The iodine/starch complex has energy level spacings that are just so for absorbing visible light- giving the complex its intense blue color.

Because the product of the reaction between iodine and starch is purple/blue/black.

The explanation of why it is that color and not some other color requires a deep understanding of quantum mechanics. People with that kind of knowledge about QM don't need to ask questions on YA.

Testing iodine with starch and having the solution turn purple/blue is the positive test for starch.