Three possibilities:

When the mophead blooms initially open, they can sometimes be a lime green color temporarily. This is just an immature bloom that, with exposure to the sun, will turn white or whatever its color is supposed to be.

When some non-white mopheads open in near neutral soil, the normal color may appear as a very light shade of green, pink or blue.

As pink/blue blooms mature and get older, they begin a plethora of color changes that is unique for that type of hydrangea. Many mopheads that are a shade of pink or blue will turn green when they get "old"; after that, they may add some dark pink/red/purple splotches; and finally the blooms may turn brown.

Enjoy the show!