A maricultured acro that is brown/grey is in a tank that's either unstable, new, or doesn't have the right equipment to keep maricultured stuff. Or it was bought from a box that didn't ship well or sat to long, but if treated right will eventually get nice color. They do not culture brown acros, they are aware no one will buy them. The people culturing these corals are aware of the fads, high end markets and know how to pick what corals they are going to culture for sale. How many stunning pieces have we seen where the seller said it was a brown piece in the corner of their tank until they had space to give it what it needed and it suddenly became a beauty?
Though i do agree with your point, you shouldn't buy maricultured stuff thinking it will be exactly like a said coral because they do change colors in a tank and under different lighting.