If you open your normal printing program for colour photographs, etc, then under the FILE column you may possibly find SET-UP where you can set up your printer to use either RGB or CMYK.
I'm assuming that your printer has two cartridges, 1 for RGB and 1 for Black??
I recently found that if you print a colour photo using only Gray scale then the blacks have a greenish hue as the printer is trying to get a black using only the RGB cartridge but if you convert the photo to a Gray scale using a colour program menu then the printer will only use the black cartridge.
Using CMYK should give you truer blacks in a colour photo (I think!)
Hope this helps in some way.