The color ink cartridge is expended even though I print in black only. Why does this happen?

To keep the print head clear and ready to print, the printer uses a small amount of ink from all the cartridges whenever it prints. Even if you select Black/Grayscale, some color ink is still being used.

Epson printers contain a permanent print head that needs to be charged with all colors to work. Even when users 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.