I know that for most audits (Such as those for BRC, SQF, GFSI, etc), you are not allowed to have any pockets above the waist. (That might be considered a GMP as well, actually.) As long as its not above the waist, then its okay. Also, the color of the coats really depends on your Color Coding policy, if your company has one. If you establish that within Zone A, they only wear Blue Coats, while Zone B only wears Red coats, then it is expected that you follow that procedure. The reason for Zoning in such a way, and therefore requiring different coats, would be for visual aid in segregation of specific zones, or products, or allergens, etc in order to prevent any occurrence of cross-contamination and/or cross-contact.
So lets say you have Zone A, which is meant for cooking Raw meat, and then you have Zone B for packaging that meat, it would make sense to have employees wear separate colored smocks or coats in order to visually differentiate between the different zones, and know who is supposed to be where at any given point in time. Or, say for example, you have an RTE packaging room and a non-RTE packaging room. You would need to be able to tell the difference between employees of the differing rooms quickly & efficiently, so maybe the best way to do that is by Color coding - including with employee use of smocks or coats.
One more example would be that you are segregating allergens between production lines. In order to know what allergen is being handled on which line, and who is supposed to be where at any given time, its easiest to know that Red means XYZ allergen, and Blue means ZYX allergen. It all depends on your color coding system, if the company has one. (Which, i recommend having one).
That's just my understanding of it. I hope this helped.