I don't know the specifics, but I can give you a general idea about why some compounds are colored.
When light shines on a substance, some, all, or none of it will be absorbed, and the rest will be reflected (or transmitted). White substances reflect all visible wavelengths of light, while transparent ones transmit some or all wavelengths.
Colored compounds absorb visible wavelengths of light. The color you see is the complementary color of the color that is absorbed. The color absorbed is determined by the wavelength of light that is absorbed. The wavelength that gets absorbed will be that which has exactly the right energy needed to excite electrons in the substance in question. Generally organic molecules with conjugated pi systems are those which are colored.
The experiment I think you are doing is a classic one:
The analysis of household laundry bleach is undertaken by adding potassium iodide to acidified bleach and titrating the iodine formed with a standard solution of sodium thiosulfate.
http://chemmovies.unl.edu/chemistry/...le/SS052c.html
I'm not sure exactly what species are absorbing light, and I don't have time to think about it too much right now. I might be able to get back to it later. In the meantime, why don't you add to this thread what the reactions are that take place and what color the solution is after each step?