Well Paul, what do you mean by their canon on color?
Theoretically, there's 3 "Naval Trooper" uniforms in the film, though the one's been debated. I've seen a picture (Here comes a blue/brown coat debate!
) of a DST standing in one of those circular control stations in the shot where Tarkin/Motti blow up Alderaan. He's in a grey tunic like most Imperial Officers (can't see a rank badge though), with the black DST helmet.
Could he be a Naval Trooper Officer? The equivalent of the Stormtrooper Officer who Veers told, "You may start your landing", perhaps?
It seems to me also that Naval troopers can don the helmet, or not, and if not then wear the black kepi. This uniform is either shared, or these men all share the same branch of service within the Empire, as Stormtroopers seem to be commanded by men in black uniforms (tunics/jagdpurs or Slacks), which is also one of the canon Imperial Naval Trooper uniforms.
That's two uniforms. The third is a seemingly black jumpsuit ala the Imperial TIE Pilot's, or Imperial Gunners suit, but with the black helmet. These guys were on Endor, and even though military continuity fell apart, they can still be fit in easy enough... Perhaps this is a uniform the Naval Trooper dons in a situation where he may get more "dirty" with his mundane job? I wouldn't say they're a different level of rank, or a different type of Naval Trooper myself, but just maybe working a grittier job than a guy who has guard duty in the detention center.
I personally believe the grey uniformed one is an officer then... A bit higher in rank, and that'd explain why he's so close to Tarkin and the gang. I'd imagine if he didn't have his com helmet on, he too would wear a black cap as well, making for a very interesting looking color combination. The Imperial grey uniform with a black kepi. Sweet!
Kepi/uniform colors DO seem to have some relevance as well, as some bridge crewman in the grey coveralls are seen with black kepi's, while others wear the darker grey officer kepi like Piett's hat, or Ozzel's. Why, I'm not sure. Could be a couple reasons.
Then the guys in the jumpsuits are a mystery unto themselves... The jumpsuit transcends Navy and Army as well, though only the grey one it seems. Black jumpsuits seem strictly stuck to the Navy (DST's, Gunners, TIE Pilots), while black tunics is unknown. Maybe they adhere only to the Navy and Stormtroopers?
The other thing then is the uniforms of officers... There's pants over the boots, pants tucked in. There's straight-leg pants, and jagdpurs... There's the black pants/white coat (and I assume black kepi) of ISB... There's the debate if the uniform is more a green than grey (I lean more towards a greyer color rather than olive, but I can see a HINT of green. A very German Wermacht WW2-ish look). Then if you count EU (And I'd like this to fit in myself) there's also a mustard brown uniform... I push these guys as being STRICTLY army unit officers of some sort, but it is indeed an EU outfit).
But back to the grey jumpsuits... There's them with black and darker grey hats. There's guys who seem to do menial technical jobs, while others obviously do bridge crew work (bud there's also guys in some kind of black uniform that work the bridge as well... Who/what are they?).
Then of course there is also the use of the Jumpsuit as the AT-ST Driver or in EU, what is known as an Imperial Army Regular... AT-ST Drivers are just a separately trained troop then? Other ideas think the AT-ST Driver and AT-AT Driver share a uniform and their differences (helmet, etc.) are the differences in a "harsh environment" suit and one that is not... not sure how I feel about that. I think they're separate personally, but I'm just not sure either.
Imperial uniforms are plain, yet incredibly diverse. I dig that.