Good observation.

The blue in V. sororia does indeed vary, but I confess I have not studied it. I cannot even say whether it varies during the life of an individual flower or from plant to plant, or from flower to flower for any individual plant.

However, I can contribute:
- I have seen intensity/shade variation in individual flowers on a red form of V. sororia, as they age
- I have seen a particularly electric-purply blue flower on V. subsinuata (close to V. sororia), but only one year

It would be good to have some observations from other folks in the US - since they have the potential benefit of larger numbers of V. sororia plants - and hence greater statistical significance. Given a good sized colony, I'd be looking to see if any colour variation seemed to be correlated with shade/sun, dry/wet soil, rich/poor soil, as well as the acidity already mentioned.

Thank you for your response. The most intense colors get the most sun, now that I think of it. I imagine the best way to do this sort of study would be in pots, controling the acidity, moisture, and other factors you mention. Right now I'm involved in a study of winter hardiness of roses in northern zones, but another year I might investigate this. Hope to hear from others.