Sharon1130,
The majority of the time this is environmentally caused. Although there are some cvs that are grown specifically for blooms that change color during their brief lifespan. These will be one color the first day and another the second. Or even change as the day progresses. But the likely cause you are experiencing is environmental. Light, temperature, humidity, soil ph, and nutrients all play a part. For me...If the plant looks healthy, and the leaves show no real sign of distress, enjoy the show.

Probably a change in the plant's cellular pH. I left this about hydrangeas somewhere else - it's a copy/paste job & might be slightly off topic, but you should be able to get enough out of it to find your answer.

"Three different pigment groups - chlorophyll, flavonoids, and carotenoids - mixed in different proportions, give color to plants. By mixing and matching the three pigments, a near endless variety of colors can be created. E.g. most reds are the result of mixing orange carotenoids with magenta flavonoids. Cellular pH (not to be confused with soil pH) can have a profound effect on plant color.

Even most experienced gardeners think that lowering soil pH produces blue blooms in hydrangea. Technically, it is only a part of the equation. Lowering soil pH makes aluminum ions more readily available for plant uptake. The aluminum is then available to bond with pigment compounds (anthocyanin, the colored component of flavonoids) changing the way color is reflected. Our perception is, blooms changing from pink to blue.

The anthocyanidin group is what makes apples, autumn leaves, roses, strawberries, and cranberries red. They make blueberries, cornflowers, and violets blue. They also make some grapes, blackberries, and red cabbage purple. One of the things that changes the color of anthocyanins
is the level of acid or alkali (the pH) in the cell surrounding the pigment. As cellular pH increases, the pigment changes structure and reflects different wavelengths of light. The anthocyanin reflects bright pink in acid cell environments, reddish-purple in neutral and green in more alkaline cells ...."

Al

I have a yellow hibiscus with a pale pink center and during the colder months it turns to a pale coral salmon colour.
Both flower types are beautiful.
I also noted that in summer the flower tends to drop much earlier than the one in winter - it's because of the heat.
Will post a picture of the coral flower soon as it's budding at the moment.