Comets are very dark gray in color, to the point of being almost black. The ion tail of a comet has a bluish appearance, while the dust tail is white.
The most accurate view of the color of a comet came from the European Space Agency's Rosetta spacecraft at the end of 2014. It was the first spacecraft to go into the orbit of a comet, and was able to send back to Earth the first true color images of the surface of a comet. The comet was called 67P/C-G.
Scientists had expected the comet to be gray, but for that gray to have a light blue hue because of the suspected presence of ice on the surface. According to the pictures sent back by Rosetta, this is not the case. Instead it is almost entirely dark gray.
The two tails that a comet has are the most visible parts from Earth. They are mostly made up of dust and gas. The ion tail is formed when ions are blown away from the comet surface by solar winds. This tail has a blue appearance. The other tail is the dust tail, which is made up of dust particles. It looks white and is much brighter.