The fading of zinnias, in my experience, depends upon the variety. I guess it could be the extreme heat, though it seems like I've had some that kept their brilliant hues in the hottest part of summer. Do you remember the variety of zinnia you've planted?

I agree that the fading of zinnias depends upon the variety. Some Profusion colors fade very quickly. All of the really dark colors, purple, plum, cerise, etc, become lighter with age. Oddly, some of the light colors get darker, but less brilliant, with age. In some zinnias, like the variety Exquisite for example, the fading phenomenon can be kind of attractive. I just wish the really dark colors would stay dark.

ZM
(not associated with any product or vendor mentioned or linked)

I can't seem to find the little tags that I ALWAYS save in a box. hmmmmmmm At least I know it CAN happen.