Why have the blue blooms on my hydragea turned green?

Question

It is in a large pot. Still in the original soil it was in when I purchased it. It was beautiful bloom now it is green.

Answer 1

I bought Hydrangea "Blue Steel" several years ago. It was a lovely deep Blue.
however, the following year it slowly reverted through Blue to Purple to Pink.
I returned to the garden centre and they told me I needed to use Hydrangea Colourant, water only with Rain Water, and plant it in Ercacious compost.
I created a raised bed, filled it with Ericacious compost, planted the Hydrangea, sprinkled the colourant granules around the base of the hydrangea and then added the colourant to a watering can full of water every 2 weeks.
Do this every year to maintain the Blue colour.
It's worth it.