According to Michael A. Dirr in "Hydrangeas for American Gardens," Cardinal Red AKA Cardinal has flowers of vibrant red in alkaline soil, blue-purple in acid soil. Leaves are dark green and lustrous, but the plant has minimal cold hardiness.
you need to change the alkaline level of your soil if you want it red. You should probably add lime. Or try forever and ever red sensation. mine is blooming a beautiful red now and my soil tends toward acidic.
If you live in a hot climate, it is unlikely you will ever see a "true red" hydrangea. No matter how convincing those pictures in the catalogs are or how much lime is added to the soil, one can only achieve a very deep or dark pink, but not a true red (at least here in the South. I'd love to hear from you if you have a different experience).
One can rarely change the intensity of a color (how strong or pale the color is). The intensity develops for a number of reasons: the heredity of a particular hydrangea variety, weather conditions (hot or cold, humid or dry), health of the plant, and possibly other natural factors. Fertilizing hydrangeas once or twice a year may result in a little more saturated color simply because the health of the plant may be improved.
A few varieties of hydrangeas tend more toward the pink or the blue range of colors, but will not retain even this color if soil conditions are not right.
For hydrangea blooms to be pink, the plants must not take up aluminum from the soil. If the soil naturally contains aluminum, one must try to keep it away from the hydrangea's system. Following are a few tricks that might work:
Add dolomitic lime several times a year. This will help to raise the pH. Shoot for a pH of about 6.0 to 6.2 (If it goes above 6.4 hydrangeas may experience an iron deficiency). Since hydrangeas take up aluminum best at lower pH levels, raising the pH will help to keep the bluing effect of aluminum out of the hydrangea's system.
Use a fertilizer with high levels of phosphorus. Phosphorus helps to prevent aluminum from creeping into the system of the hydrangea. Choose a fertilizer close to the ratio of 25/10/10 (Phosphorus is the middle number).
In areas that naturally produce blue hydrangeas (soils with aluminum), consider growing pink hydrangeas in large pots. If hydrangeas are grown in pots, it would be best to use soil-less mixtures, since these mixes would probably not have aluminum in them. In a pot, it will be much easier to control the requirements for growing pink hydrangeas.
It is most diffenantly hot and humid here,I have huge oaks and pine trees in my yard so soil is very acid. I put lime around all my hydrangeas last year but not enough because I didn't see the desire effects this year.