I bought a Canon Pixma MP620 when I got my 17" MBP last summer. I *very rarely* print color but just noticed when I tried to print a b&w document that it is warning me some ink cartridges are low. I understand the black since when I print I always select greyscale, but shouldn't that mean I don't use my color cartridges? I printed a full page color photo about 4 months ago but I think that was the last time I ever printed color. When I got the warning today I looked at my ink supply levels and the black, blue, and magenta were under the line indicating needs replacing. I would assume at some point while printing in the past those color ink levels must have actually passed through that line mark. Why wasn't I warned then? Is there something I should know about printer ink supply warnings?