Dell 720 won't print color, color ink cartridge missing

I just bought 2 dell ink cartridges, once i installed them, i tried them out with a test print. A message appears saying an ink cartridge is missing, which happens to be the color ink, after i make the adjustments and put in the settings that both cartridges are new, i print a test print, and the only ink appearing in the black one. Basically the color ink is non existant. Help!!.. i'm a college student and i need to print many things out ~COLOR...!!