Thanks for the question.

The actual color of hydrangeas is often dependent upon environmental factors. As advertised, your hydrangea blossoms probably started out having a pink hue and then gradually became white. It sounds as if your blossoms are now turning green. This greening is seen in most varieties of hydrangea so your case is not unusual. Perhaps not widely known, the color seen in hydrangeas is not due to its petals but to its sepals. Whereas petals are integral parts of the actual flower, sepals are part of the stem. It is the color of these sepals that determines the color you see. These sepals contain a number of plant pigments including chlorophyll (green pigment). Early in the season the chlorophyll levels in these sepals are low and the various other plant pigments come through. The actual colors that emerge can depend upon the soil pH. However as your hydrangeas grow and mature during the season, chlorophyll increases in these sepals, becoming more and more prominent, and so the hydrangea blossom assumes a green tint. There is little you can do the prevent this transition. It occurs sooner in some hydrangea varieties and later in other varieties. It is just part of the plant and quite normal. See:

https://www.provenwinners.com/plants/hydrangea/invincibelle-wee-white-smooth-hydrangea-hydrangea-arborescens

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/green-hydrangea-blooms.htm

https://www.plantindex.com/hydrangea-flowers-turning-green/

Thanks for asking us this question and please enjoy the ever changing nature of your hydrangeas.