because if you run the color heads dry it will fry the heads and send an error message. The ink works as a coolant to keep the heads from over heating.
Fill it with water, or cut the color print head ribbon and tape both ends of it so it won't arc. 2 ribbons go into the head carriage, it's the one closer to the BACK of the printer. Then you can crack open your cart and remove the bag itself...no more error message. You can do that with all of your colors as well.