Why were all the basic imperial uniforms starting in The Empire Strikes Back gray?

Question

In A New Hope, John Mollo's Oscar-winning costume designs included imperial uniforms in a number of different shades. Besides the most frequently occurring gray, there were also plenty of officers in black, and (during the meeting scene) one person in a much lighter tan.

However, in The Empire Strikes Back and Return of the Jedi, these alternate colors are never seen. It cannot just be a matter of different services (imperial navy versus army), since when onboard Vader's command ship, General Veers wears the same gray uniform as the naval officers. (Veers has an entirely different outfit when he goes into battle on Hoth, however, and my avatar shows actor Julian Glover in another SF costume.)

Is there known reason why the alternate uniform colors were discontinued in the later movies of the original trilogy? The field gray Mollo chose is certainly reminiscent of Nazi uniforms (in the same way that the bridge layout on the star destroyers are suggestive of slave galleys).

However, the SS also used black uniforms, which are probably equally iconic.

So what is known about the reasons for this change? Was it just part of a greater standardization of the imperial uniforms? (For example, from Empire onward, the colored cubes on the left breast were always in two rows, with blue above and orange below, while the came in a wider variety of patterns in the first movie.)

Answer 1

Short answer: they weren't discontinued.

Long answer:

It all depended on what they were, stormtroopers, tie pilots, prison guards, etc...

From Wookieepedia (the article cites six different Star Wars sourcebooks as its sources):

Army and Naval Officers

• Olive grey double-breasted dress tunic and trousers
• Corresponding cover
• Corresponding Battle Dress (Armor, Helmet and black goggles with utility buckle, black belt and duty holster)
• Black leather belt with silver buckle and optional duty holster
• Black leather boots (durasteel-capped)
• Black leather gloves (optional)
• Rank badge on left breast
• Code cylinders located in pockets

Officers in the Stormtrooper Corps, Prison guards, and veteran TIE pilots

• Black double-breasted dress tunic and trousers
• Corresponding headgear
• Corresponding body armor for combat use, just like any other trooper (sometimes with the addition of colored shoulder pauldrons as high-visibility rank indicators when in field units)
• Black gauntlets (optional, as seen on Death Star)
• Rank badge on left breast (without armor, as seen on the Death Star and the Tantive IV, and with Snowtrooper armor, as seen during the Battle of Hoth)
• Black duty belt with silver buckle and utility boxes with optional duty holster
• Code cylinders located in pockets

Death Star Troopers/Naval Troopers (SNCOs) Senior Non-Commissioned Officers

• Black trousers and double-breasted tunic
• Black gauntlets
• Black open-blast helmet
• Black duty belt with silver buckle; utility boxes and holster
• Black boots
• Firearm-DH-17 blaster pistol (around the Battle of Yavin)

Imperial Gunner (Enlisted)

• Black overalls
• Black gauntlets
• Imperial emblem on each upper arm at the shoulder
• Black enclosed targeting-computer helmet with one white Imperial Emblem on the front
• Black duty belt with utility boxes

Vehicle Command Crews (VCCs) & Senior Enlisted Crewmen (NCOs)

• Light gray single-breasted overalls
• Olive cover
• Battle Dress helmet with black goggles (VCCs- AT-ST Drivers at Battle of Endor)
• Imperial emblem on sleeve at each shoulder
• Duty belt
• Vehicle Command Crews and NCOs were of any rank above corporal

Technicians (Warrant Officers & NCOs):

• Light gray single-breasted overalls (NCOs)
• Black single-breasted overalls (WOs) with black gauntlets (optional)
• Black cover
• Imperial emblem on sleeve at each shoulder
• Duty belt

Not listed on the page linked but there's also the Grand Admiral uniform that was all white with gold epaulets and a black belt, black gloves, and black boots.

As far as the bars worn on the left they were the imperial rank bars:

A lot of the characters that appear in the movies would be NCO's or others that would wear grey, whereas in A New Hope there were more scenes on the Death Star with higher ranking CO's and more shots of gunners, prison guards, and other types of personnel that would wear different colored uniforms. In The Empire Strikes Back there wouldn't have been as much call for say, Stormtrooper Corps Officers out of their armor or Death Star Troopers or Prison Guards to appear on screen.

Here is a shot of the bridge of the Executor from Empire Strikes Back with at least two, maybe three, different colors represented:

Here are some of the different colors represented in Return of the Jedi when the Emporer is arriving on the Death Star: