I bought a couple of hydrangeas last week - one is a deep red colour and
the other was a beautiful deep blue.

There's no problem with the red one but, in the space of a week, the other
one has gradually changed from deep blue, through purple, and is now a kind
of washed-out pinkish-blue which doesn't appeal to me at all.

I'm not a gardener, but I assume the acidity of the soil has something to
do with this? Is there anything I can do to restore it to the beautiful
deep blue colour it was when purchased?

Failing this, if I bought another deep blue Hydrangea are there any
precautions I can take to prevent this happening again?

TIA.

--
Exiddor.