Did something happen to change the PH of the soil. Like hydrangas other plants will change color if the soil PH is changed.

Did you amend the soil with anything. Peat moss will make the soil acidic. Check back and remember what you put in. You can make it more acidic by adding acidic fertilizer and less by working in agricultural lime in the soil around
the plants. Be careful not to get the lime on the plants or roots because it will burn

Did you spray any herbicides nearby?

too hot maybe?

I find that 'Rozanne' flowers vary in appearance from day to day and from morning to evening. Sometimes they look very pink, but usually they are on the blue-r side. Maybe temperature and humidity affect the color as well.

When it's hot, my 'Rozanne' is pinkish purple.
It turns almost indigo blue when the weather turns cooler in the fall. I find that quite interesting.

Bigleaf hydrangea is the only plant that responds to soil pH by changing flower color. Color variation in the flowers of other plants is due to exposure (sun/shade), climate conditions and the age of the flower. There IS a color variation in the flowers of 'Rozanne' depending on the existing climate conditions and time of the season - cooler temperatures seem to encourage the blue tones and hot, sunny weather bleaches the flowers out to a more purply-pink color.