HP Officejet Pro L7650 Print head problem
The reason why the printer refuses to print if you get low on one or more colors is that the system runs off negative pressure through the ink lines to the print head. If you were able to continue printing with an empty cartridge you could get air in the respective ink line, disrupt the negative pressure, make it so that that color will no longer print, and require you to get the printer serviced to fix the problem. I own a Cartridge World ink refilling store and I feel this is a good machine (compared to some of the others out there). The only down side with this type of machine is the print heads expiring or getting clogged as you are experiencing. The reason the black is clogged and not the other colors is that the black ink is pigment based and the colors are dye based. The pigment based inks do tend to clog print heads over time. You can try to rehydrate the print head with warm distilled water. if you can't get it working to your satisfaction you can buy a new black/yellow print head for $72. With this type of printer you should print at least one black and white and one color document per week to keep the print heads fresh. If they sit too long without ink flowing through them then they can get clogged and print poorly. I hope this helps belay some of the resentment you hold towards your printer.