Absent an employment contract or union agreement to the contrary, a business can set the conditions of employment much as it sees fit, so long as no form of legally actionable discrimination plays a role. Accordingly, if your company has a uniform, your boss has every right to insist that you wear it. Further, the term “uniform” is often a broad one. It entails any article of clothing that identifies the employee as working for the employer. That having been said, requiring a plain white undershirt will most likley not be condsidered to be a uniform as it can be worn off hours and is typically worn anyway.