I don't think that is what anyone said. If you use only 6500K lighting your corals will grow well but will be brown. The nice colors are pigment to block UV
light (we tan brown, corals tan fluorescent colors). By using actinic or higher kelvin color lighting you will provide the light that will make the corals increase their coloration.
This is why reef keepers use 10,000K lighting or higher, usually supplemented with actinic.
In addition 6500K will ook very yellow on a reef tank.