I never use any colour other than black yet I have

You don't have to replace the coloured cartridges if you only use black. But if you print something that is not pure black then it will use the other colors. For expample, if you print on a gray scale, since this is a color printer it will use the colored cartridges to get the gray color. The colored printer will only use the black ink for pure black colours. If you just use black to print, why not use an inkjet printer that just prints black or uses black cartridge. It is available in the market.