Coffee is not a permanent dye and will come out when your hair gets wet. Because of this, it may also stain clothes, furniture, and pillows when you sweat. It is best to throw a towel over your pillows before lying down to protect them from stains.
Also note that coffee is not ideal for every hair color. If your hair is naturally blonde, coffee may not be dark enough to produce a pleasing color, instead leaving a muddy hue. And if your hair is dark already, you may not see any difference at all. Coffee might be best for lighter brunettes who want to go just a couple of shades darker. It can also be ideal for natural brunettes who are going gray and want the gray strands to more closely match the rest of their hair.