I found this:

http://www.flowersbulbs.com/ql_hydrangea_color.htm

but it may not answer your specific question. The impression I get is that the intensity of Hydrangea coloring--as opposed to the hue--is pretty hard wired into the genes of the plant, rather than being very dependent on soil chemistry.

Try giving it some Lime. Hydrangeas can be made to turn color (blue) by adding some (Ferrous Sulfate) this acidifys the soil, Or( Pink) by adding (Lime) to make the soil more alkaline.

Agree with Tugbrethil - the colour of the flowers is largely down to the genetics of the variety you've chosen - the exception is with blue flowers, which often turn lilacy pink when grown in alkaline conditions. If you want a darker reddish pink, choose a different variety.

Adding lime will tilt a lavenderish Hydrangea more towards the pink end of the spectrum, but it won't make it a deeper pink.

So how does it happen that a usually pink Hydrangea has this year grown deep blue and bright pink flowers on the same plant?
This has happened to a few of Mums, they are in a clay soil, have been there for many years and apart from pruning in spring have had no other care.
Sorry to but in, just very interested.

How soon after they were planted did this happen, Seabird - the following year? And is there any difference in height between the pink and blue parts?

I don't know when they were planted but it was before 1987 and it's happened this year. They are very old and very hardy, they don't even get watered in a drought and there are trees nearby! The leaves are droopy with lack of water but the flowers are bright.
We're not worried about them, they have survived worse and keep coming back every year.

Thanks to all that replied, I will try adding a little lime. I have a deep blue hydrangea also a white one I just wanted to try and make my pink one a shade or two deeper. Many thanks again.