Hello lora11,
Thanks for using the Apple Support Communities! I understand that your iPhone 11 Pro Mac is "stuck" in grayscale mode. I can definitely see how this could be concerning. You have come to the right area and I'm happy to help!
The first is to check the Color Filters for Grayscale like you mentioned that you did. Go to Accessibility > Display & Text Size > Color Filters, toggle Greyscale off/on.
Next, which is less obvious is within the Zoom settings. Check to see if this is turned on in
Settings > Accessibility > Zoom > Zoom Filter and select 'None'.
Cheers!