Just for future reference, color is a matter of perception; it's not necessarily a molecular property. Concentration can affect perceived color quite dramatically as can extrinsic factors such as the ambient light.
The definitive way to see if the optical properties have really changed is to take an absorption spectrum, and keep the conditions as identical as possible: concentration, solvent, pH level, and so-forth should be identical. If at that point the spectra are different, then likely your sample has changed: either you have an impurity, or the sample has undergone a reaction (oxidation or polymerization, most likely).