Why do I need color ink cartridges when I print in black only?
The printer will not print if any ink cartridge is expended or removed. Epson printers contain a permanent print head that needs to be charged with all colors to work. Even when you are printing black text only, a small amount of color ink is used to keep the print head charged. The print head needs to be kept charged to prevent air bubbles from entering. Air bubbles can damage the print head.
Therefore, once one of the cartridges is expended, the printer signals you to replace the cartridge in order for printing to continue.