Why does iodine change color on certain food?

Pure iodine is violet. It can act as an electron acceptor (an acid/ oxidizer <-- because it is taking negative charges (aka electrons) from something, leaving that something with an overall positive charge). When iodine is in a solvent that doesn't donate electrons it stays violet. When it is in water or ethanol the iodine accepts electrons from the oxygen atom which then affects the wavelength of visible light that it absorbs, this means that the colour changes.