There are several variables that can cause color shift, alone or in combination - temperature, sunlight, nutrition and pH. https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-turning-different-color.htm

At this point, I am blaming it on sunlight, or lack of sunlight in December. I've looked at older photos I have, and I did not a slight shift in color in previous years, mostly November through February. I don't have photos from previous Decembers, and so maybe in past years, they did not bloom as much in December. I do think the color will shift back to yellow in the spring now.

Hibiscus colors tend to vary a lot. Single flowers can change color through the day, usually becoming paler as the day goes by. Temperature will also affect how they bloom. Some cultivars will have more spots and splashes in hot weather. Others will be more reddish or darker in cold weather. This is because heat tend to destroy some of the red pigments.

My temperature does not change much from summer to winter, and so I think sunlight is a bigger factor. Some of my plants will have differently colored flowers at the same time, and I have no explanation for that. I haven't noticed the neighbors' plants being affected as much as mine, but they tend to have red hibiscus instead of yellow. My hope is that the flowers will return to yellow next spring. We often have very cool and overcast weather in June (locally known as "June Gloom"), but I have not noticed that having an effect on the color of the flowers.

Genetics and cooler weather can affect pigment production.

Hereafter "resists" changing from yellow (somewhat) during winter... at least near Temecula, CA.

Lars, it would seem that my multi-shaded hibiscus had reverted to yellow for it's first flowers. I hope that it goes back to the multi-shaded.... orange fading to yellow throat.