Why Does a Colour Ink Cartridge Run Out Faster?

We sometimes hear from people that print almost exclusively in grayscale or black and white when they have unexpectedly run out of colour ink despite using it only very rarely. While it is true that, left unused long enough, some ink types degrade and stop working there is a simpler explanation. Many printers run short tests every time they start up before printing and part of this testing process is to ensure the flow of ink. To do this the printer releases a small amount of ink from each cartridge into a test pad that remains inside the printer. Done enough times these tests will deplete the reserves of colour ink cartridges even if they are never actually used for printing.