While the color episodes were often very lush and beautiful (especially when set in period times, particularly with the costumes), the black-and-white were far more atmospheric. Black-and-white, too, seems more "forgiving" regarding special effects - for example, the ghosts appeared more "3-D." Also, during the black-and-white period, the exterior filmed shots were used, giving the show a unique quality, along with more sets and background performers giving it a more realistic approach. Something was lost with the transition to color.
Gerard