Last year I used hydrangea colourant to turn my deep pink mophead hydrangea blue. I diluted it in accordance with the instructions and applied it once a week from early May to early July. To my disappointment the flowers turned out mauve-purple rather than blue.
I tried again this year in the belief it would take another season to make the soil acidic. But this summer the flowers were actually pinker than last year!
Where did I go wrong? The only thing I can think of is that I didn't fork around the roots before applying the colourant.