Hi I purchased a blue one from my local garden centre and it started turning pink but i then went back and bought some hydrengea blue colour pellets that you scatter around base and water in and it has come back blue

You don,t say whether it is still in a pot or if you,ve planted it out, if it,s planted out then you must have alkaline soil, which will make the flowers pink, give it a feed of either sequestrine, or aluminium sulphate, and the flowers should turn blue again, you will have to do this annually to keep the flowers blue. derekm