sorry for updating after for so long, if you look at the buses
with white rims you can notice always at the rear wheel the rims is
always dirtier compared to the front wheel rims.
I guess it's because the rear wheels have a higher chance of
contact with the kerbs when the bus enters the bus-stop. Also, soil
particles and mud/gravel on the road can also soil the wheels
easily. Hence, I feel white rims are pretty difficult to
maintain.