All hydrangea blooms turn different colors as they age. The most common color for pink and blue hydrangeas to turn is green (especially in the South where climates are hot and humid). After the blooms turn green, they may pick up shades of pink and burgundy.
Annabelle hydrangeas, which bloom white, ALWAYS turn green when they have been in bloom about two weeks (sometimes they stay white a little longer).
When hydrangeas have aged and turned different colors, one can pick the blooms for dried arrangements (see: Drying Naturally). Dark, unattractive petals can be snipped out with scissors.
[NOTE] Occasionally hydrangeas that are normally pink or blue will bloom green in some years. No one seems to know what causes this, but often it occurs for only one or two years and then the blooms return to their normal color. Using a fertilizer that contains trace elements may hasten the return to a normal color.
In cooler climates (and occasionally in hot ones), some hydrangeas age to beautiful shades of blue and purple. As far as I know, there is nothing we can do to promote these lovely colors other than to keep the plants well hydrated.