Why do they wear white and the other Team Rocket members wear black?
Team Rocket, and what they wore in only the first few episodes of the Black/White series (on the right):
What they usually wear:
Is there a explanation for this?
There is no official confirmation about the rank vs uniform colour meaning but it's speculated that White uniform is for B-Rank agents.
The first kind of black uniform is worn by Team Rocket Grunts.
The white uniform is worn by "B Rank" agents. Mondo wears this uniform, and Jessie and James wore this until Memories are Made of Bliss!, and then again in Battling For The Love of Bug-Types!.
The second kind of black uniform is worn by "A Rank" agents. It is worn by Cassidy, Butch, Jessie, James, Domino, and Miyamoto. It is similar to the black uniform worn by grunts, but instead of black gloves and boots, they wear white gloves and boots with a red stripe around the top.
The silver uniform is worn by "A+ Rank" agents. Attila, Hun and Tyson wear this uniform.
The uniforms worn by trainees are identical save color, one is dark red and the other blue. In Training Daze Jessie and James wear the dark red uniforms and Butch and Cassidy wear the blue. There does not seem to be a difference in the trainee uniforms as far as gender is concerned.
The only time Jessie and James wear black costume is when they were promoted in Memories are Made of Bliss! by Giovanni. Why they changed their costume back to white is never confirmed. It was different fact that Giovanni had no expectations from the team except he knows their presence can make Team Plasma to reveal themselves.
It's because they are the visual definition of "don't do that" in every corporation identity guidebook. Black uniforms are wear by grunts and "A rank" units (with white boots and gloves for A rank) White is for "B rank". So Jessie and James finished their grunt training and moved to B rank but never get to A rank (like Butch and Cassidy)
Jessie and James usually wear white uniforms for two reasons. The first, which is out of the Pokemon universe, is that the creators of Pokemon wanted Jessie and James to be unique, as lots of Team Rocket members wore black uniforms. The second reason, in the Pokemon universe, is that Jessie and James are B rank agents and all B rank agents wear white uniforms. They changed their uniforms back to white because they thought that Giovanni had lost faith in them.