Hydrangeas are fascinating in that, unlike most other plants, the color of their flowers can change dramatically.
It would be nice if one could change the color of hydrangeas easily. But for most of us, it is not easy. The people who have the most control over the color of their hydrangeas are those who grow them in containers. It is much easier to control or alter the pH of the soil in a container than it is in the ground.
On the other hand, hydrangeas often change color on their own when they are planted or transplanted. They are adjusting to the new environment. It is not unusual to see several different colors on one shrub the next year after planting.
It is much easier to change a hydrangea from pink to blue than it is from blue to pink. Changing a hydrangea from pink to blue entails adding aluminum to the soil.
Changing from blue to pink means subtracting aluminum from the soil or taking it out of reach of the hydrangea. This can be achieved by first treating the soil with a phosphorous based fertilizer followed by a treatment of lime or calcium. the pH should be raised to 6.0 to 6.5 but never higher than this.
That said, I’ll give the best information that I have on this subject and let you take it from there.
In order to change the flowers of the hydrangea from pink to blue there needs to be aluminium present in the soil and a pH of less than 7 ideally between 5.2 and 5.5 (Acidic). Only plants older than 2 years which are established should be treated. Before commencing treatment to change colour it is strongly advisable to water plants thoroughly every day for a week. Where possible test soil around hydrangeas for pH.
Add 15g (1 tablespoon) of aluminium sulphate in a litre of warm water and allow to sit for 15-30 minutes to dissolve. Add this to a watering can and make up to 5 litres with cold water. Apply the solution around the based of the hydrangea. DO NOT OVERUSE as making the soil too acidic can result in damage to the roots. Check the pH and maintain it between 5.2 and 5.5. Other simple things you can do include adding as grass clippings, coffee grounds and fruit or vegetable peelings to help lower pH levels. Potassium rich fertilizers will also help to achiev this.