What are 'single colour' cartridges?

For "single colour" cartridges (also known as separate ink tanks) you will need to buy a full set (one cartridge for each colour, of Cyan, Magenta, Yellow and Black, or CMYK for short, K is blacK). If you are a heavy user of just one colour (your business logo for example might be blue) then when you print your letterheads you will use more blue (and probably some black) than any other colour. This means that you will be able to just replace the blue (Cyan) ink cartridge rather than replacing all of the colours at once. If you had a cartridge with all of the colours in one then you would not have this benefit.