faulty ink

I have the HP 7160 that I got for Christmas last year in 06. I really liked the idea that it came with different color ink cartridges. I normally only print out research papers that are just black, and within a few months is said that the yellow was almost out, which I thought was very odd, I changed it and still continued to print black and white research papers, and again a few months later it said to replace the magenta cartridge...now its saying to replace the blue cartridge. I feel like its programmed to say you need to replace ink colors that haven't even been used to sell more cartridges. Very dishonest.