Can a landlord restrict what a tenant’s employees wear for a uniform?

As a general  matter, the landlord could only restrict what your employees wear if the lease itself gives him some authority or power to do so. That said, there are a few other situations which could give the landlord the right to demand a change or take action, such as:

1) If the tenant misrepresented the uniforms to the landlord and they are not what the landlord was told prior to agreeing to rent, that misrepresentation could give rise to a right to rescind the lease, if the tenant does not correct the problem. Whether or not its a misrepresentation depends on more than just the actual uniform--for example, if the uniform would be acceptable if worn closed, but the employees will wear it open, that failure to state the way the uniforms will be worm could constitute a misrepresentation.

2) If the uniforms are such as to "disturb the quiet enjoyment" of other tenants (e.g. there is a nursery school or religious-affiliated charity also in the same building, and the uniforms are incompatible with those other tentants), the landlord may be able to demand that the situation be corrected (or else potentially evict).

3) If the uniforms violate any local ordinances or other laws, the landlord is not required to expose himself to potential liability over them, but may demand a change.

So generally, the landlord could not restrict a tenant's employees' uniforms, but there are some situations which could allow him to do so.