Based on observations and no hard evidence from a controlled experiment...and Mike, if you do decide to try what you mentioned, I'd suggest you need to find a way to keep the water pH neutral otherwise it becomes an uncontrolled parameter....I have to think that pH has no effect on flower color.
I have seen lilies in bloom from New England to Florida and Florida to the Pacific Northwest and the colors have always been true to type, but I never asked what the pH of the water was at any of the farms/nurseries/botanic gardens I visited. Still, I have to think they encompassed a range of pH from somewhere less than 7.0 to at least the 8.6+ of south Florida water.
Personally, I have had blue/purple lilies turn pink....'Midnight' in particular...but when I repotted the lily in the same type soil, placed it back in the same water at the same temp, they invariably reverted to back to purple. The only difference was that I had added fertilizer to the pot and have to assume that a lack of nutrients was the cause.
Other circumstantial evidence....Hydrangea are one of the few flowers to change between blue and pink, so it is not an expected occurrence. And the blooms on Hydrangea are blue at low pH and pink at high pH and even then it is a reflection on the uptake of Aluminum and not the pH per se. A hydrangea grown in soil lacking Aluminum should be pink despite the pH ( I think<g>). Whereas the lilies I have grown are blue at a high pH with fertilizer and I assume,as my water flowed constantly, they were also pink at the same pH when lacking fertilizer.
Finally, in the book by Charles O. Masters 'Encyclopedia of the Water-Lily', he expounds quite a bit on soil and water conditions and the only reference to pH being a factor is that he found lilies grew best ( in natural waters) at a pH range of 6.5- 8.6. The only effect he mentioned related to low pH was that lilies in oligotrophic, acid waters tended to be smaller. He mentioned nothing to do with pH and color.
So my saying pH has no effect on flower color is intuitive and not based on any hard evidence....more on a lack of evidence.<g>