I believe that is what they're supposed to do...my favorite thing about hydrangeas.

Purples will fade to an almost blue and white will fade to a pink.

What i have found with my hydrangeas is they change their color if the soils ph level changes or if the soil happens to become more acidic mine have stared to change into blues or if aluminum was accidentley added they turned pink.

Alter your pH, if needed, by treating the plants in spring before they bloom. Lavender to purple flowers are produced with a 5.5 pH. If your flowers are usually blue, or your soil has a low pH, then you will need to raise the pH. For pink flowers, or a soil with a high pH, you will want to lower the pH.

Raise the pH, when necessary, by introducing lime into the soil. Applying 4 oz. of lime to the base of the plant can raise the pH by one point. Use either 2 or 4 oz. of lime in each application to slowly raise the pH until you reach the 5.5 pH required to see purple flowers.

Lower the pH, if needed, by adding aluminum sulfate. Follow the package for instructions on using aluminum sulfate carefully. Usually you can use 1 tbsp. per gallon of water around the base of the plant for each application. Because aluminum sulfate can burn the roots if used incorrectly, you may opt to change the pH by adding organic matter, such as coffee grounds, homemade compost or grass clippings.

Allow at least 2 to 3 weeks between applications of lime or aluminum sulfate to really give the plant time to adjust to the new soil and change colors. Pay attention over time to how your plant is doing and whether or not your small changes are taking effect. No matter what you do, you will not see instant results, but it is possible to overdo it in the waiting process.

you can buy soil testing kits at Wal-Mart for around $17.00

Ummmm......jadedlotus, I think you have this process somewhat backwards :-)

The coloring of Hydrangea macrophylla flowers are determined by the available aluminum in the soil. Acidic soils (low pH soils) are more inclined to release or make aluminum available, resulting in blue or purple flowers. Soils with higher pH (neutral to alkaline) limit or restrict aluminum availability, resulting in pinker flowers.

Applying either lime (to raise pH) or sulfur or aluminum sulfate (to lower pH) and influence flower color will not have any rapid effect - it will take 6 months to a year to see results, due to both the necessary chemical reactions these must go through in the soil and the plants' bloom patterns and susceptibility to color change.

Since these are potted plants and not in the ground, the coloring was very likely heavily manipulated by the grower, as forced hydrangeas undergo a rigorous regime of fertilization and growth hormones and that is hard duplicate in the home environment. It is also very likely the irrigation water is influencing the color change, not the soil. Watering with a dilute vinegar solution or MiracleGro for Acid Lovers (aka Miracid) should help to maintain the necessary acidity of the water.

Age of the blossom will affect coloring......whites take on pink tones and blues and purples tend to fade with age. And there is nothing that can be done about that except hope the plant will produce new flowers. Not always a sure thing with any florist or greenhouse grown hydrangea, or one that is in bloom now (March 1).