"Have an HP printer set to greyscale to conserve the color cartridges. Color cartridges are still being depleted. Why?"
Because HP's printer software is designed to lie to you. This is pretty much an industry-wide problem; almost all printer companies sell their printers at or below cost, and make their profits by selling grossly overpriced ink cartridges. They deliberately design the printer software to tell you that you're out of ink long before you actually are. Some of them will actually stop the printer from printing, even if it has enough ink!
The printers also have sponges inside which are solely designed to soak up ink and waste it. Every time you clean the printer head(s) (as the software often recommends), it simply squirts a large amount of ink into those sponges.
I have an Epson printer. I hardly ever use it. Because when I do, it tells me that I'm critically low on ink, and tries to force me to buy new ink - from Epson. There's a box to turn off the low ink warning, but it's purely an exercise in frustration; it turns itself back on EVERY TIME! I hate Epson (which is ironic, since I used to be a fan).
Kodak recently started trying honesty with their customers (http://printandprosper.com
). Their printers are now more expensive, but the ink is reasonably priced - about $10 per cartridge, I believe. That makes a lot more sense to me. I hate not letting my son print out his pictures because I'm afraid of running out of ink. The whole point of having a printer is to USE it! So when the ink finally runs out of my Epson, I'll be buying a Kodak.
By the way, someone may suggest that you take out the ink cartridges and reinstall them. Unfortunately they have magnetic codes built in which your printer will recognize; it will not reset the cartridge life, and will probably refuse to print altogether!