My calendar colour keeps changing to a colour i don't want.

Hey there shelo9,

That is a strange thing to occur, and we'd like to help. Let's begin with confirming more details to better understand what's happening, along with ruling out some possible causes:

1. When did this start to occur, or has this occurred ever since you attempted to change the color for one of your calendars on your Mac?
2. Have you noticed this only occurs for one specific calendar for which you change the color, or does this occur for any calendar?
3. Sometimes unexpected behavior can be resolved with a restart. Check out the section ‘Restart your Mac' here: Shut down or restart your Mac
4. We see you have macOS 11.7 installed, but the latest version for that older OS is macOS 11.7.3. You can verify this by clicking on the Apple logo at the top left corner of the screen, then on ‘About This Mac’. In case you needed it, this link shows how to first back up, then Update macOS on Mac.

Cheers!

Hello shelo9,

If this continues on multiple devices after performing all those troubleshooting steps, then we would advise contacting Apple Support next. They would be the best suited to investigate this behavior further. Contact them here: Contact - Official Apple Support

Take care!