The presence of aluminum makes the sepals turn blue. Hydrangeas absorb aluminum far easier if the soil is very acidic, which makes it water soluble and easily absorbed by the plant. So by increasing the acidity of your soil, you get better blues. By making the soil more alkaline, the shrub cannot absorb aluminum as well as it used to and you get lighter blues, purples or pinks. Near the point of neutraility, you get lighter blues (they are sometimes light blue; light blue + green; almost white) or purples. In alkaline soils, you get shades of pink.
Purples are incredibly difficult to achieve because they occur in a narrow area of soil pH readings near the point of neutrality. Pinks in acid soil can be achieved by amending the soil with garden lime. This makes the soil more alkaline. A soil pH Kit is necessary to monitor the soil changes. Because of the buffering tendency of the soil to revert back to normal, you will have to apply these amendments "forever". But do not bother trying to do this during winter if you have snow cover or if it is too cold outside; that is just not practical.
Note that it is difficult to change and control the acidity when the shrub is planted in the garden. You can end with one side more or less acidic than other; a container setting is easier. This difficulty results in shrubs that have blooms of various shades of blues-purples for example.
Hydrangeas that have white blooms will not have bloom color changes if you alter the soil. Only colored ones do. The best pinks are obtained around pH readings of 6.5 to 7.0 in the soil pH Scale. My soil is already alkaline so I do not need to do anything. You may do well with one or two annual applications of garden lime per package directions. When working with lime, it is important not to go overboard as it can cause iron chlorosis! So keep an eye on the leaves. This is an example of iron chlorosis... thanks to Colorado State Univ:
You can try to ahieve good purples using mopheads such as Geoffrey Chadbund, Hornli, Blue Wave or Blue Bird.
Does that help you?