The activity bands just turned all white today. Was fine yesterday. How to return color to bands

I remember another user who reported a similar problem recently. I think the fix was found that somehow the Greyscale filter had been activated on the Apple Watch (Settings, Accessibility). You might notice some other things have also lost colour if this is the case

See details for where to find the setting in Apple support document Use Accessibility features on your Apple Watch - Apple Support

The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support