LRC 12.4 - Colors gone in develop mode for Canon CR2

Did you perhaps activate one of the healing tools, with 'Visualize Spots' checked in the lower left corner below the image?

The first thing that you should try is to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue. 

Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom. 

https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html 

Another step is to try to reset the Lightroom preferences. 

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/ 

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html 

If this doesn't hellp you should also check your monitor profile.

https://www.lightroomqueen.com/how-do-i-change-my-monitor-profile-to-check-whether-its-corrupted/

If this also doesn't help we need more informations about your environment. 

Please provide also a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. 

There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.