Hi Rkp52, reading up on it, there doesn't seem to be a great deal you can do apart from dead-head regularly and hope that the new blooms will be stronger in colour. The older heads will naturally fade over time.
I thought it might help by changing the pH of the soil, but apparently not - anything between 4.5-5.5 will create blue blooms, below that in more alkaline soils are the pink bloomers, but altering the soil to increase the alkalinity wouldn't help either I'm afraid.
So advice would be to fertilize twice a year, dead head and try not to position them in too much sun.
Not very helpful I'm afraid, but it might might some difference, all the best