Five Starbucks in Colorado have joined the effort growing across the country to unionize the omnipresent coffee stores with the readily recognizable green signs featuring a two-tailed mermaid. Two stores in Denver, two in Colorado Springs and one in Superior have filed petitions with the National Labor Relations Board to be represented by Workers United, an affiliate of the Service Employees International Union. The momentum started in December when employees at a Buffalo, N.Y., Starbucks voted for a union.