Hi steve—f,
Welcome to Apple Support Communities. From my understanding, colors on your iPad Pro appear unexpectedly when using Sidecar. I’d love to assist you.
First, consider whether this happens when viewing multiple apps and websites. Also, try using Sidecar in multiple lighting conditions and when connected to multiple, reliable networks.
Additionally, be sure that your iPad’s display isn’t in Night Shift or using another display accommodation when this happens:
Use Night Shift on your iPhone, iPad, and iPod touch
Use Display & Text Size preferences on your iPhone, iPad, and iPod touch
From there, check to see if this happens when your iPad Pro is connected to another Mac. This step helps isolate the issue to your iPad or to your Mac.
I hope this information is useful. Kind regards.