QPrinter : "GrayScale" should not be included in the supported color modes for macOS

Request from a commercial user :

"Printing on Mac always prints in ColorMode"
https://bugreports.qt.io/browse/QTBUG-36856

This bug report is saying "Unfortunately manually setting Color Mode is not supported under OSX".

Actually if you try to set the color mode to "GrayScale" on Mac OS, it doesn't work.
It's just ignored, and there is nothing to notify of the failure. 

Although it's Showing "GrayScale" as one of supported color modes, selecting it as the color mode is rejected without a notification.
This is confusing for users.

It would be good if there were a way to notify the aplication of that the attempt was failed, but QPrinter::setColorMode() does not have a return value.
Please consider another workaround, for example, hiding "GrayScale" from the supported color modes.