HP 3740 series - Manual (Page 10)

Installed print cartridge Result
Black print cartridge.
Colors are printed as grayscale.
Tri-color print cartridge. Colors print but black is grayed and is not a true black.
Exiting ink-backup mode
Install two print cartridges in the printer to exit the ink-backup mode.
For instructions about installing a print cartridge, see the
print cartridge installation
page.
HP Deskjet 3740 series printer User's Guide
10