These need ericaceous soil, is yours in that or, is your soil acid ?
They will always revert to pink if the soils ph isn't acid enough and no matter what you do the soil is the only real factor, you can add allsorts of copper items but it's not enough, they need acid soil - the garden centre should have told you this when you were buying/looking at it.

Can you dig it up (DO take the entire rootball) and create an acid bed there ? Pop it back in and with some luck it'll slowly colour up blue again.

If I'm not mistaken, you mention a pot - so if its in a pot, go out and buy some ericaceous compost, and repot into that instead.

Ah, right.
I hope it's a decent sized pot :-/

Thank you for asking this question! I thought I was going mad. I only moved into my place last year and could have sworn my hydrangeas were blue, now both are coming out pink. I know what to do now.

Thanks all x

You can buy a compound from garden centres that will help them stay blue. Also you can water them with Epsom Salts. Mulch with rotted lawn mowings and all the coffee grounds you can lay your hands on as both these are slightly acid and will help a bit. My soil is only just on the acid side of neutral and mine stays blue, so it doesn't have to be a very acid soil.

Aluminum sulfate is the compound sold to turn Hydrangeas blue. Note that Epsom salts (magnesium sulfate) will turn some varieties of Hydrangea white!

Sequestrene iron tonic works as well.

My Hydrangeas are blue and I wanted to a dark red one. I wonder if I can put something that can make them red?

No you can't, Aimankay - blue hydrangeas don't stay perfect blue in soil which isn't acidic - the others are largely unaffected. Grow yours in alkaline conditions and they'll be pink or lilacy pink.

Thats amazing, Tug! Never heard that before.

Well I missed that completely - when you say turn white, Tugbrethil, I take it you mean the flowers, not the foliage?

The flowers, yes. I haven't found a complete list of those it has this affect on, but it is mainly the ones that are sold as white, but tend to turn patchy light pink, plus some of the ones sold as light pink or blue, and a few of those rare "picotees"

Good to know -nothing more annoying than a white hydrangea turning pink. A member on here had slight pinking on a white one, if I can remember who it was, might be worth his trying this.