They change color due to the acidity or alkalinity of the soil they are grown in.
you need to change the soil PH, if i remember corectly you need to lower the PH but ask a nursery :)
The colour of hydrangeas is affected by the pH of the soil - soils with a low pH (up to about 5.5) are acidic soils and will produce blue or purple flowers. Soils with a high pH (above 5.5) are more alkaline and flowers tend to become pink as the pH increases. Many modern Hydrangeas are bred to be more inclined to either blue or pink, but in my experience they usually eventually revert to the above 'rule'.
Hydrangea flowers blossom into several different colors. Included are: bright pink, light pink, light purple, periwinkle blue, yellow, and ivory. Some even contain two different colors on the same blossom in a pink/light green combination, a pink/white combination, and a purple/blue combination.
they change colour with the soil pH level.
yes
I posted a link that has everything you need to be succesfull in you Hydrangeas planting in the related links box below.
In gardening
No.
This plant is not fussy about fertilizer types. Use whatever is inexpensive in granular formulation, such as 10-10-10, applying it as per package instructions at the end of winter.
Aqua blue.
Hydrangeas