Uniforms do this today and I would expect the human experience of donning one would still be significant even in the far future among our interstellar cousins. One isn't a Marine or a police officer until they put the uniform and insignia on for the first time.
And you reconcile that statement with this how?
EXAMPLES:
In Zulu War, Brit army officers wore red or blue as they wished.
AVG had no uniform, pilots wore what they brought and many had a uniform tailored up in Rangoon.
Few navies had Enlisted uniforms prior to mid-19thC and officers tailored uniforms were not uniform to one another, merely similar.
US Army in Indian Wars kept issuing either Civil War or lowest bidder items that wore out; so from 1866-83 mixed uni-parts and civilian clothes worn on campaign, uniformity slowly growing after '83.
The SS issued the fez to Balkan muslim units in two colors red/feldgrau to keep them happy.
Age of Enlightenmemt armies frequently had different uniforms per regiment in style occasionally in colors, some of which carried forward to the 20th century in both field and dress uniforms.
You will note all my examples are not from times known for "tolerance and diversity"
In modern times our military bends over backwards to accommodate religious needs of Muslim troops. They're in demand especially as translators.
Now to the Imperium. It is vast but it has no coercive powers over its citizens that current Earth governments use. The best example is that the Draft is voluntary, none serve unwillingly. So when wars and problems occur the Imperium needs bodies it needs a ready supply of them, so if a planet with a population of 10 billion won't wear green, that might be a problem for the army and marines recruiting drive. The imperium isn't going to write off 80-90% of the recruiting base on this world and a dozen others like it, they will accommodate it and come-up with something else. The Vilani ran a cookie cutter society, it was too fragile to withstand a big pressure, The 2nd Imp was (unintentionally) too freeform, make it up as you go along, it collapsed into anarchy. The 3rd Imp has no intention of following those 2 down the tubes. Hence it encourages wildcard, the "traveller" culture to keep on movin' cross-polinating the Imperium with fresh ideas, hires them for nasty odd jobs or tosses them on a prison planet to relieve them from the stress of petty larceny on occasion. Yet it embraces the slow change of the old 1st Impie that had no use for vagabond travellers spreading their non-conformist ways.
So why does no one agree on 3rd Impie uniform colors? They have multi versions, eliminating a potential source of nonsense before it ever happens.