I agree with scampi, to use smocks, lab coats, hair nets, etc., to make outsiders highly visable. Really it should be part of your food defense/food security plan. The whole concept of uniforms as a GMP is to minimize new and different contaminants from coming into your facility. Certainly every facility has its own native "dirt" but the point is you don't want employees, contractors or visitors of any category bringing in new and different filth and allergens and Pest. At one IPM short course I took, it was explained that most institutional/facility infestations are caused from employee belongings. Even if you have a uniform program, but allow employees to wear it to the lunch room or out to lunch, then you have negated the purpose. What if they go out and pet contaminated with peanut allergen from that cany bar orThai food peanut sauce? Then they come back from lunch and pose a dire risk. You have no clue where the sales representatives coming to your facility have been before they come to see you. They could have walked through a barnyard or the pigeon and duck mess at the park. My point is a uniform program is not complete without a shoe/boot program as well. If you send individual workers from a zone 1 or 2 area of exposed product out to fetch boxes or labels from a zone three area, then they will bring contaminants back to your most vulnerable area. This is why you should have disposable aprons, sleeve protectors, etc., in addition to uniforms to minimize zone to zone contamination. Uniform programs must also have a strict prohibition of wearing or taking uniforms into restrooms and providing ample facility and procedure for accomodation of that policy.
One of the most difficult chalenges to a uniform program are posed by the uniform vendors themselves; many do not have progarms, certifications or any warantee whatsoever concerning the efficacy of their washing, sanitizing and protection from contamination for the uniforms they provide. A qualified uniform vendor needs to provide their customers with supporting documentation. Without the science based data and certification from an acredited body, you have no clue if the uniforms are providing any more protection than if your employees were wearing their street clothes in your processing room.
Uniforms are notorius for having unwanted pockets, loose buttons and threads that get into food. If you don't insist on a contractual specification to prohibits buttons, pockets and a criteria for wear/damage replacement then you create a new problem of contamination.