Calender Colour changes randomly

for the last 2 weeks (around the time of the last osx update) iCloud randomly resets the colours of my calenders. These changes are then forced through to my Mac and iPhone. I know this seems really trivial, but the bright yellow colour that it resets my work calender to is really hard to read, particularly on smaller screens. anyone else experiencing this issue or know of a way to make it stop? i tried writing to apple support, but got no response.