The next shoe is dropping in an effort to end what is described as decades of persistent and systemic bias at the agencies that create, buy and place television commercials and other forms of media advertising. The N.A.A.C.P. is calling on the nation’s 25 biggest advertisers — on TV and in other media — to require that their agencies use racially diverse teams of employees in creative and account-management posts. The first letter in the initiative is to be sent to Procter & Gamble, the No. 1 advertiser in the United States with ad spending last year, according to the...