World Color operated a printing plant. a written policy stated: "Baseball caps are prohibited except for [world Color] baseball caps." The policy permitted employees to accessorize their uniforms "in good taste and in accordance with all safety rules." The Graphic Communications Conference of the interna- tional Brotherhood of Teamsters filed an unfair labor practice charge with the NLrB claiming that the policy prohibited employees from wearing union insignia at work. it said world Color was interfering with its employees’ "right to self-organization, to form, join, or assist labor organizations, . . . and to engage in other concerted activities for the purpose of collective bargaining" as guaranteed by federal law. The NLrB agreed and world Color appealed. was the baseball cap policy an unfair labor practice?