In-universe that is. Out-of-universe explanation may just be that it sticks out against the black Vader / backgrounds.
White strikes me as an especially poor choice of colour for uniforms. Except for snow, it sticks out like a sore thumb.
Is it because, like the British redcoats, they engaged in a lot of melee combat where it was important to easily see your friendlies in chaos? Was white an important part of Empire symbolism?
Short answer is that it isn't always white. We do see in episodes I, II and III that other colors were used to better blend with other environments.
However, white was the primary color as listed at Wookieepedia.
"having acquired its color from the austere tastes of its Kaminoan creators"
Wookieepedia - Stormtrooper armor
Additionally on the Stormtrooper Wookieepedia site, the White armor signified unit solidarity and became a distinguishing signature characteristic. As it was illegal for anyone other than a Stormtrooper to own the white armor it became indelibly linked as the armed face of the Empire.
However, I would suspect, while not specifically stated, it is likely that white as a color would also be the most likely to resist blaster/laser fire by being the most reflective. Hence, could be a good default color choice particular in an interior ship environment.
In episodes IV, V & VI we see pretty much just white. We see them on ships, a very hot desert planet (again white might provide a cooler alternative) and snow. Definitely what could be considered "white" advantage situations. However, outside those situations any additional possible benefit of reflective capability would be offset by the non-camoflauge capability. Too, whether it violated the "austere tastes of its Kaminoan creators" if that lack of "blending in" was getting Stormtroopers killed, even the Empire would need to rethink things :-)
Again per Wookieepedia:
"Stormtroopers sometimes wore camouflage armor to help them blend in with different environments. The Stormtrooper corps instituted this reform after the embarrassing defeat at the Battle of Endor."
Hence, for episodes IV, V, & VI, we might consider "white only" version as an anomaly. After getting clobbered in VI, we can see that restriction was discarded in favor of the most optimum color armor for the job.
These are great answers in-universe.
However, one can argue that it comes down to cinematic visual cues.
What's a good bad guy army? Intimidating and inhuman.
If you look at the armor, it's basically a futuristic, stylized walking skeleton. Even the face of the armor is a lamenting skull. A very simple, yet effective idea.
Film as a visual medium, benefits from showing as much of its content in as little dialogue as possible. Stormtroopers needed to give the impression of intimidating, inhuman killing machines at first sight.
It's hard to argue that encasing your big bad's legions in armor shaped like naked Grim Reapers and giving them guns that shoot blood-red fire bolts was not so they could be frightening.
Might not have been a good choice for real-life application, but it got the message out to the audience.
I think it is a number of answers mainly to do with the fact that white represents the fact that they are mindless drones and the whole good bad guy thing i think is to do with the second world war as that is what hitler was doing and he had some soldiers called stormtroopers.
The reason why Stormtroopers cannot have customized armor is because, during the old republic it was a tradition to have markings and symbols which Palpatine banned at the start of the Galactic Empire. He wanted Stormtroopers to look more sophisticated and more professional, instead of having them have there own designs.
