Just wait until the printer halts in the middle of a job - it will beep three times and you will see that the whole icon for that particular cartridge is flashing. Open the ink cartridge access lid, pull out the old cart and put in a new one of the same color.
The machine will then continue printing again with no further input by you. Miraculously, there will be no discernible artifact in your print as a result of stopping and starting again. A truly amazing piece of equipment. Initially I used to open up my empty cartridges just to see if they were really empty - they were.
