Why is my colour ink almost as empty as my black ink despite only printing black and white, and having the ink choices set to print in grayscale only?
I am using a Canon MG5400 printer.
While what Jan is saying is true and applies to some printers, the really big differences come from elsewhere:
1. Colour cartridges hold far less ink than black ones (usually about half, sometimes as little as a quarter).
2. When printing a text document, most of the printing will typically be in black. Page coverage (the amount of the page that's actually covered in ink) is typically around 15-15%. When printing in colour, however (whether photos or colour graphics), coverage is far higher, up to 80-90% for many photos, so a single A4 photo can easily use up aropund ten times the amount of ink used in a text (mostly B/W) document! Furthermore, the colours on a colour page are mixed, so there's a certain amount of overprintinng (the same dot on the page being covered by more than one ink), so the cyan, magenta and yellow may well exceed the total coverage of the page.
3. Ink cartridges age and dry out, whether you print or not. Some makes (e.g. HP) will prevent you from using a cartridge that is over a certain age (6 months in cases I've seen). I don't think Canon does this, but the cartridges still dry out, so over a period of time will lose some ink, and as Jan also mentioned, all cartridges get cleaned from time to time which uses more ink.
I can think of two reasons:
1. On Black/Grayscale most ink printers use the color ink to make the black appear even darker, basically they print the colors over each other and finish it off with black. Respectively, they just use some color up as "waste".
2. Whenever your printer cleans the printing heads e.g. when you change the black cartridge it will clean all of them using up a surprising amount of ink in the process.
To quote Epson:
"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 or Grayscale, some color ink is still being used"
So really, unless your printer offers a true "black ink cartridge only" mode it will always use up some colors during printing and will always use up color when cleaning the heads.
That's the business model of ink printers: You get a photo quality printer for $100 bucks but new cartridges will cost you another $80.