While some printers absolutely demand that you change the color cartridge, with others, there may be a way to print that important document now, even if you can't get to the store until tomorrow.
Open your printer driver (you can usually get there by clicking on "File/Print..." and then clicking on the button labeled "Properties"). Look for a tab labeled "Color" (or something similar). Find a check box or radio button that says "Print in Grayscale" or "Print in Black" and choose it. If there is a button on the same window that says "Default," choose that.
What you are trying to do is set the driver so that your printer will only print in black. When you do that, it's very possible that the printer will not stop you from printing even if the color cartridge is empty.

