Experts and BHG readers answer.
Thanks for writing. There are no hydrangeas that have true, fire-engine red flowers... Most are a shade of reddish pink or reddish purple.
Hydrangeas are a bit of an oddity in the flower world, too, because the pH of your soil infuences their bloom color. Acidic soils bring out blue tones and alkaline soils bring out pink tones. So if your soil is on the acidic side, the blue tones could be keeping it from being as rich of a red-pink as it could be. (If you have acidic soil, adding some lime will help bring up the pH.)
It's also possible the plant was mislabeled at the garden center. Some varieties have the darker red-pink flowers; others are more of a bright pink.
---Justin, Senior Garden Editor, BHG.com