Diffusion and Osmosis of Starch, Iodine and H2O?

I know that the dialysis membrane is permeable to iodine because its molecules are small. For example: When H2O and Iodine mixes together in the environment and the tubing is inside filled with starch, it will turn purple because of the reaction of iodine and starch. But how come when the environment is a mixture of starch and iodine, the tubing filled with H2O doesn't turn brown. Since I know that starch can't p through but iodine can and when H2O reacts with iodine it turns brown. Though, how come the distilled water didn't change color?