You dream in color simply because that is how nature has been since forever. With the introduction of black and white television, the mind was exposed to the possibilty of a monochromatic enviroment. Since those older people were a part of that time in history, it has made more of an impression on their minds.

I'd also bet the farm that should we ever create a holographic enviroment where everything was semi transparent, you would start having dreams that way. Most likely it would be quite often during intial exposure and the decline with time.