Green no longer works on Calendars

Hello,

I recently updated my iPhone and iPad software to the latest iOS. Since doing so, the calendar that I had marked in green font is now in purple/pink. Even when I change the colour back to green, all the entries quickly revert to being purple/pink. The same is true on my Macbook pro. I have tried changing it on all the devices but they quickly switch back to purple.

Has anyone else had this problem?

Any bright ideas for how to permanently fix the problem and get back to being able to use green?

With thanks in advance,

RB