We see here and here questions about wearing a black hat. However, why is the hat black? Is there some sort of kedushah to wearing a black Stetson or Borsalino which is lost in other colours? Or is there some other reason for people having specifically black hats?
But hats were not always black. See, for example, this picture of Yeshiva bochars (boys) of Maytchet and this picture of the inauguration of the Yeshiva Beis Yitzchok in Kaminetz where black hats are definitely a minority.
So unless it's a throwback to the late Middle Ages, the insistence on black seems to be merely a fashion.
Many things we take seriously today had very humble beginnings.
From a purely practical perspective, black clothing requires less maintenance. In times before quick, cheap, efficient cleaning1 laundry was a major undertaking, and some outerwear (like hats) didn't like soap and water. Walking around a European city that could politely be described as "filthy"2 while wearing white will have you looking like a tramp by lunchtime. If you wear black, people can't see the dirt. Thus, black becomes popular, and is subsequently made to be fashionable (yes, this skips a few steps, like the wealthy being able to afford the cleaning).
In Victorian London (MAJOR dirt problem) residents went as far as keeping black chickens. Just as nasty as white ones, but you couldn't tell just by looking. The Prime Minister's door at 10 Downing street is traditionally black. Today it's painted that way, but before WWII it was because they gave up on constantly cleaning it.