How exactly do printers get dark colors?

as a printer engineer, i can happily say that 600x600 DPI should be plenty enough for good quality prints. large A3 scale Laser printers that print at 600x600 are fairly good quality, provided the original being printed is of good quality. when you scale this down for 6x4 for example, this should provide plenty good enough quality for you. the reason why colour has to be much higher is that it "overlays" the dots of colour over each other in order to create the correct colours for you. what is the model i may have one i can lay my hands on to check it out for you!