dress (up)

[CaseyA]

Suppose workers at a factory change into special uniform before working. Would I write:

The workers are dressed in uniform.

or,

The workers are dressed up in uniform

[SoothingDave]

The workers are dressed in uniforms. Or, more simply: The workers wear uniforms.

[emsr2d2]

I certainly agree that it's "dressed" not "dressed up". However, I don't know if this is a BrE vs AmE difference but I would use the singular "uniform".

The workers wear uniform.
The workers are in uniform.
The workers dress in uniform.

[SoothingDave]

I would say they are "in uniform," but for the other phrasings I would definitely use the plural "uniforms."