Hydrangeas are in full bloom right now, 
and some of us may be wondering why that beautiful pink shrub that we planted last year has now turned a bright shade of blue! Hydrangeas may produce pink, blue, or lavender blooms, depending on where it’s planted and how it’s fed. The presence of aluminum in the plant ultimately determines the color, and pH affects the uptake of aluminum. Alkaline soils, pH of 6.0 or more, are more likely to produce pink blooms, and more acidic soils, pH 4.5 to 5.5, produce blue flowers.
Pink hydrangeas can be turned blue by 
applying aluminum sulfate to lower the pH and add aluminum to the soil. Applying lime to raise the pH level will help blue hydrangeas turn pink. If your soil naturally produces very blue or very pink hydrangea flowers, you may need to grow your hydrangeas in containers or raised beds to achieve the desired color. If you do attempt to change the color of your blooms by adding these minerals, dilute them well, and add sparingly. It is very easy to scorch your plants by adding too much. White hydrangeas are not affected by efforts to change bloom color.
Over the years I have sometimes adjusted soil pH. Not very hard to do. Trouble is I don’t always know whether I like the pink or blue flowers more!
I have a big Penny Mac that solves the dilemma–some of the blooms are pink, some blue, and some a combination of colors. Experiment a bit and see what happens with your hydrangeas.