If you’ve ever had an Epson printer, it’s quite likely that at some point in time you will have attempted to print in black only to receive an error message that you cannot print until a colour cartridge is replaced.
Why can't I print In black when a colour is empty?
and I quote….
“If ink is not in the tubes from the head to the cartridge, air will be sucked in and therefore cause damage to the printhead. This is because the nozzles are in the printhead and not in the cartridge.”
What Epson are basically saying is… if this safeguard was not in place, and you sent a print request for black when a colour cartridge was empty, there is a risk of air being pulled into the printhead. This air will damage the vacuum needed for the printhead to pull ink in.
Additionally, if any air is allowed to get into the aforementioned tubes from the head to the cartridge, any remaining ink will coagulate. This coagulation may lead to you not being able to print again with that cartridge, or to a blocked nozzle/printhead which requires a significant amount of cleaning cycles to resolve.