I feel silly asking this question because I thought I knew all about this. A friend of mine told me that water is actually blue. i.e. its blueness is not merely a matter of preferential scattering (the usual explanation), but that the chemical that is water is actually (very faintly) blue. He said that the colour comes from the interactions of light with the atomic (or was it molecular?) bonds. He also said that water is virtually unique in this regard, all other substances that have colour do not have it this property. This is very much against my understanding but I have a lot of faith of him.