A lot of inkjet cartridges have memory chips inside the cartridge which the printer uses to keep a record of the number of prints that cartridge has done. Then when the printer estimates that the ink is getting low, it writes some data into the cartridge that says the cartridge is empty. I've opened these empty cartridges to find that they're still 1/4 full so obviously this is all done by the companies to get you to buy more cartridges (which is what they make their money on, not the printers). That's why I threw out my fairly-new inkjet printer and bought a laser. I now use the machines at KMart or BigW to print out in colour because it works out cheaper.
As for the pictures printing out in different colours to what's on the screen, this can sometimes be due to differences in the colour calibration between the screen and printer. That's why they sell fairly expensive devices that stick on the front of the screen and calibrate the colours so that when you print, What You See Is What You Get (WYSIWYG) (I think I'm showing my age by knowing that term).