Even though they have about the same mass as our own sun, they are roughly 40 times smaller. Brown dwarfs are really large planets like Jupiter. Since they aren't stars they don't shine so astronomers refer to them as being brown, since they are very dark. Here are some additional comments provided by Phil Plait. I have a special page on my web site that talks all about star colors.
Hot stars are bluer, while cooler stars are red.
A red dwarf is a tiny red star, pretty much like the Sun but much cooler and smaller. They are very small, about the size of the Earth. Brown dwarfs are a newly discovered type of object, which are midway between stars and planets. They are too small to be real stars, but too big to be called planets.