How to change the color of your garden hydrangeas
Hydrangeas are a very unique flower as you can actually change the color of their blooms. With a few changes in fertilizer you can change your pink flowers blue or your blue flowers pink. You can also deepen the colors with other fertilizers. The white hydrangea however does not fall in the color-changing category and will always be white. Sometimes as the bush ages it can turn the edges of the flowers pink.
Pink to Blue
Changing the blooms from pink to blue is fairly easy, you just have to add a little aluminum to the soil. This is done by treating the area the plants are in with aluminum sulfate, about a tablespoon per gallon. This can be purchased at any feed and seed store that carries chemicals. Then regularly fertilize with one high in pottasium, such as (25/5/30). To enhance the blue of an already blue flower fertilize with powdered pottasium or a fertilizer with a high 3rd number.
For an organic approach, composte full of organic matter, coffee grounds and worm casings is high in aluminum and lower the pH over time. A pH of 5.2-5.5 is optimal.
Blue to Pink
Turning your blue blooms pink are a little harder to do because you have to remove the aluminum, or raise the pH of the soil to achieve the coveted pink blooms. The chemical way is to use dolomitic lime and fertilize regularly with a fertilizer high in phosphorus and nitrogen. Make sure the middle number is high for phosphorus or a powdered variety is available. Phosphorus prevents the plants from absorbing aluminum and keeping the color you desire. A pH of 6.0-6.2 is optimal, much higher the plants can get iron defficiency.
A more natural way can be to heavy mulch with pine bark or mulch. The nitrogen will leach out of the bark, but it will take time and a large amount of dirt would have to be replaced to achieve a color change. There are also natural sources of phosphorus to be found.