Recently, I printed out a possible button design which had nice distinct colors, and it was obvious that my majenta cartridge wasn't working.
It hadn't occurred to me to consider whether the problem was with my inks because the printer refuses to work at all when one of the ink cartridges runs out. In fact, it nags me about it if a cartridge is running low.
However, this doesn't keep it from getting a clogged nozzle. A couple of rounds of deep cleaning--I wonder what the printer is doing when it rattles through that routine, and I had a nice image of Saturn in stripey shades of peach and black.
Fiddling with more maintenance got rid of the stripes, though one of my adjustment test pages doesn't seem to produce all the colors. On the other hand, printing a rainbow (conveniantly found by google images) showed that my blue cartridge is still printing, though perhaps not as brightly as it should be.