Merck KGaA announced that Douglas S. Brown has been named president and CEO of EM Industries, Inc. He replaces Tom Colclough, who retired June 30.      Mr. Brown joined EM Industries in September 2001 and has more than 20 years of experience in specialty chemicals, including a number of senior management positions with Ciba Polymers and Ruco Polymers, a division of Sybron Chemicals. Mr. Brown will retain his responsibilities as head of the U.S. pigments business at EM Industries....