Hello, I have exactly the same problem
I had today a call with Lexmark France customer support.
The guy was confident and installed a new printer
Instead of selecting the Lexmark CS510 Series spooler in the dropdown list, he used the "generic PCL Laser printer"
Result : Printer can print in Black and white but the result looks awfull.
Lexmark has no other solutions to date, their drivers are outdated of 2017 and they don't even mention it anywhere on their website.
The compatibility matrix of all printers for macOS are blocked to MacOS 10.13 !
http://support.lexmark.com/index?page=content&userlocale=EN_US&locale=EN&id=OS22
If someone succesfully pass to fix the driver or have a solution to restore a color mode printing on the Lexmark printers, please let us know :)