Long time ago since I saw a printer with only the one, they usually have at the least one colour (which contains yellow, magenta and cyan) and a black, more often now you have 6 or so.
Anyways, when printing colour, unless printing yellow, magenta or cyan only then it will more than likely use some black as well and therefore probably refuse to print colour if the black is empty. Black should use black only, and in theory of course when the colour(s) is/are empty then black should print, however printer manufacturers make little profit on the printer itself, but plenty on ink so it is often the case that if one runs out then the printer wont print till you replace both (that only applies to printers that take a black and a colour cartrigde - printers that use multiple shuold print as long as there is sufficient in in them all)