A:
the Best Buy specs on this page seem to say that you MUST have 2 cartridges inserted in order for the unit to function. (Specs tab, "Number of cartridges required: 2")
but i pulled up the User's Guide on HP's website, and that says you do NOT have to have both inserted.
also, maybe more to the question--Norma, you can just replace the black cartridge if the tri-color cartridge still has ink in it. you can buy, i'm pretty sure, a single black cartridge, although buying a 2-pack of BLACK cartridges might be more cost-effective.
btw, since this printer apparently DOES allow for only 1 cartridge to be inserted, i was told (long ago) that if you don't print with color very often, you should remove the color cartridge and store it in an air-tight bag (e.g., ziplock or baggie). that will keep the ink from both drying out or evaporating. when you want to print in color, just insert the cartridge.