I am appalled at the price of colour cartridges and I don't do anything in colour now that I have finished work. Actually, when I was at worked I used the printers at work.
Most printers are designed so that when one or more of the color cartridges are used up (even when not being used, the ink will eventually evaporate), they will not print even when just using black. For about 3 years now, I've been buying replacement cartridges from "Cartridge World" for half the price of Canon's. I think you have them in the UK.