If you use the Google Chrome Browser or Chromium and all Prints over Cups / Cups-PDF or Color Printers are all greyscale there is a known Bug at the Printing Menu of the Browser!
This is NOT a Problem with you Cups Printing Setup or Operating System!!!
(Alternate you can test your Cups-Printing-System with Firefox here you get color Prints)
To fix this you can try to install a other updated PPD Printer Driver which is working with Chrome / Chromium
Download here:
https://github.com/schlomo/HP-Color-LaserJet-flow-MFP-M880/blob/main/HP-M880-Chrome-Color-Print.ppd
then open a Browser to http://localhost:631
go to Printers, change the for example Cups-PDF Printer at new Model upload and install the PPD File and test.
Now you should get color Outputs.
I tested it well for Cups-PDF Printing over 2 different Cups Servers.