The same could be said for all the different legions though... They all basically look alike aside from their specialists or their commander (Cody is unique, his AIrborne troops are unique, his AT-RT Troopers are unique, but his regular troopers all look alike and the Airborne and RT Drivers all look like each other... no discernable rank).
It seems somewhere between AOTC and ROTS, either rankings are generally dropped, or their identification methods are drastically altered. There's nothing that seems to cover it but I'd guess there's something that marks the troopers that only other troopers can identify... Perhaps this came about from targeting of commanders by CIS forces?
Obi, talking with Cody prior to his departure of the ship they were on, had a number of other troopers there paying attention... Other lower level "Officers" or non-coms under Cody? Just nosey bystanders?
Tough to say really... I think rank's something that Troopers tend to only be able to discern between each other after ROTS rolls on... And that then carries over to the classics with only visible rank markings being given to units for specific reasons.
The rankings from AOTC were never very intelligently layed out or described... There were some serious discrepencies or unexplained issues. I think it just didn't get much attention paid to it in the prequals. The "Star Corps" (Bly's group), I thought, were described as sort of a special unit... Perhaps his troopers we see with him when he lays out Aayla are something akin to special forces troopers of some sort? We don't get a good look at the troopers in the AT-OT... Do they all look like who Bly's standing with? What about the gelagrub rider? Maybe who Bly has with him are other lower level officers beneath him and they have "basic" looking troopers (no Kama/pistols, no visor), AT-RT Drivers, etc., etc., that bear the same markings as the troops we see up close with Bly, just not the added gear.
Tough to say... I think there was serious lack of thorough thought on the Clones from Lucasfilm/ILM, and then from the sources left to hash through it all afterwards (books and whatnot). I had the same gripe with AOTC where there were more visible officers by the book's definition of colors and hardly any non-coms... I mean, you can count the green clones you see on one hand when they should be the second most abundant color besides white "troopers".