I have heard that is an age thing. People who grew up with monochrome TVs and movies tend to dream in black and white, while people who are used to color in moving pictures and on TV dream in color. eta: to respond to Echos, dreams are most like TV shows or moving images in our real-world experience - so it does make some sense that people's brains would adapt once they came along.

But I also would say we can't exactly tell for sure, can we? Because any colors in dreams are provided by the subconscious ("color is the pigment of the imagination" etc.), not an actual light source. In which case, black and white should also count as colors, if colors in dreams are subconsciously significant. Or, your conscious memory of the dream might be filling in the colors, kind of like if you watch a monochrome Charlie Brown, you know Peppermint Patty's hair is red, not dark gray.