Garden centres usually sell a home test kit or a pH meter.
A lower pH means the flowers tend to be blue.
A higher pH will mean the flowers produced are usually pink.
It’s not uncommon for hydrangeas to be shades of green and cream to start with.
This will change as the flowers mature.
Soil pH 5 to 5.5 = Blue
Soil pH 6 to 6.5 = Pink/purple
Soil pH 5.5 to 6.5 = Purple, or both blue and pink
To make your hydrangeas’ pink flowers turn blue, the soil needs to be made more acidic.