hydrangea colour

Give 'em a can of Iron Bru :D

Blue hydrangeas don't just need iron, they also need acid conditions and aluminium (as aluminium sulphate). Some basic info from Ashwood Nurseries should help out.

Another point often overlooked is that young plants, cuttings and plants that have been moved often revert to a pink colour for the first couple of years and some pink varieties will be pink no matter how many girders or alluminium cans you throw at them :D