Changing Hydrangea Colors You see, for most French hydrangeas (Hydrangea macrophylla), the flower color indicates the pH of the soil. In strongly acid soil (pH below 6), flowers turn blue. In alkaline soil (pH above 7), flowers turn pink or even red.Click to see full answer. In this way, what affects the color of hydrangeas?Bigleaf hydrangeas are blue if the pH is between 5.0 and 5.5. The flowers turn pink when the pH gets around 6.0. The flower color is dependent on the concentration of aluminum ions (Al+++) in the soil. Aluminum is more available to the plant when the soil is acidic, less available when the soil is alkaline.Furthermore, does Epsom salt change the color of hydrangeas? When the ions of Epsom salt disassociate, they have a neutral effect on the pH of the soil. Since the blue color of a hydrangea is formed by aluminum made available in acid soil, adding Epsom salt would not make your flowers change color. Similarly, do coffee grounds change the color of hydrangeas? If you’re growing hydrangeas, use coffee grounds to affect their color. Coffee grounds add extra acidity to the soil around hydrangeas. On a chemical level, this increased acidity makes it easier for the plant to absorb naturally occurring aluminum in the dirt. The effect is pretty blue clusters of flowers.How do I get my hydrangeas to turn back in color?Sprinkle 3 to 4 cups of garden lime around the base of the plant. Again, you will have to add the lime every 3 to 4 weeks throughout the growing season to keep the blooms pink. Now that you know how to change the color of hydrangeas, head over to read about how to care for them in this Essential Guide to Hydrangeas.