Qt allow print i the two color mode: color and grayscale. Even if select grayscale the program will print in color mode.
Issue tested with Qt 5.2.1 and Qt 5.5.1 on Ubuntu. Printer (driver) support grayscale mode. GIMP easily printed the same image in grayscale.
Maybe is this only Linux/Qt problem? Need to test the code on Windows. Can someone confirm this?