I've written an entire hub about hydrangeas "How to Plant and Grow Hydrangeas." Some hydrangeas are white no matter how you fertilize them. Some are pink and some blue, also no matter how you fetilize them.
Then there are the hydrangeas that you can change the color. Hydrangeas like fertile soil with lots of organic matter. You can provide this with cow manure , mulch and peat moss. To get a blue color, apply aluminum sulfate. To get a pink color apply dolomite lime.
For more information, you need to read my hub, which has a lot of information about when and what to use for fertilizing a hydrangea.
I guess I should have written an entire hub on how to change the colors of hydrangeas or how to fertilize them.