Starch test

[smhjn17]

I have tested for the decrease in the amount of starch in a sample of rice left at room temp for days.

I have seen significant decrase due to the decreasing intensity of iodine blue black color. Howeever, unlike the initial fesh rice the test on subsequent degraded samples shows a color say light purple which gradually fades away in 2-3 min and becomes colorless..my basic question is why in degrading starch does the color intensity of iodine test gradually change from slight blue black to no blue black??