Grey Group, the wholly-owned subsidiary of advertising agency WPP, has acquired a majority stake in digital media company Yolk Pte Ltd.Yolk is a Singapore-based group with operations in Malaysia, Thailand and Australia. It works on digital strategy, social media, online advertising and marketing for clients like Canon, Ciba Vision and Microsoft.It reported an unaudited turnover of 1.3m Singapore dollars for the last financial year. No other financial information has been given.