The hydrangea plant is something of a litmus test for soil pH. In the acid soil environment the flowers are deep blue in the alkaline soil environment they are pink. This color reation will show the sectoring of areas underneath the plant with areas showing blue and pink depending on the subterrean pH. The pigments can be sensitive to pH and color can be useful for inferring environmental pH values. Since plants are generally most productive at the neutral pH the liming according to blue flower pattern might be very useful. Plants can be wonderful bioindicators and most of our world soils are probably not optimized for critical values of soil pH.

Prof. Paul Reed Hepperly already added details explanation. Following !

The red areas of our global soil map are plagued by acid soil infertility. The acidity predominates in older soil families such as ultisols and oxisols which are red in color acid in pH and are high in metal toxicity Manganese, Aluminum and Iron and generally have multiple mineral deficiencies. When these issues are addressed they can recover their productivity. These soils are also many times very low in soil organic matter and fix Phosphorus in forms which are not easily taken up by plants.

The blue alkaline soil areas are found in dry areas where evaporation exceeds precipitation. They are many times both salty and alkaline and the remediation of these soils depends on replacing sodium salt and acidifying to near pH neutral. These soils can display physical issues from the salt and low micronutrient availability.

The majority of all soils are not optimized for their pH reaction and indicator plants are excellent means of diagnosing the underlying issues and the cost is modest compared to laboratory analysis.

Anthocyanins are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue or black. Food plants rich in anthocyanins include the blueberry, raspberry, black rice, and black soybean, among many others that are red, blue, purple, or black.

If we use our hydrangea plant as our living litmus the plant in the aerial portion is mirrored below ground.

As such the areas within the same plant with are pink will reflect a local alkaline condition within the root system and the area is blue will reflect acidic soil condtions.

This biological indicator can then be used to selectively add elemental in zone of pinkness and to lime in the areas of deep blue.

The potential importance of this is that areas of either soil acidity and alkalinity are usually compromised for plant production.

So the plant can indicate or tell the producer what its needs are in the soil and the plant and farmer can benefit from the plant giving out it red and blue flags so to speak.

Anthrocynamins are plant components which serve as signals in other ways the best food choice from that aspect are the ones with deeper and richer colors. These same indices provide the way that pollinators recognize the are to look for both nectar and pollen facilitating the pollination of mutual benefit.

Anthrocyanins are also known as phytoalexins which are important constitutients in the plant version of responsive immunity. When plant are exposed to stresses the stress can inititate the enhancement of plant pigments and this is not a pigment of your imagination it really happens.