Even though I've only been printing in black and white all other colour cartridges are running out.... how come????
The printer uses all inks to make black.
You can use grayscale but it still uses some of the color inks.
Some printers have a control setting where you can specify to only use the black cartridge but I don't think Canon has it.
I might be wrong on this so someone else may know better but I believe it to be true.