if a school Does have a colored belt system and somebody decided they want to wear their white belt I feel it is a pretension of humility and I find this type of person extremely arrogant and inconsiderate in their partnering, in general.
Hmm, I haven't noticed that in the people I've seen doing it. They get on the mat and train like everyone else. There is also a custom in our dojo that if you've been off the mat for a while you wear a white belt (even if yudansha) until you feel up to snuff again. I wear a white belt when I train at Daito ryu seminars and some others...I also used to wear a white belt at our saturday morning basics class (the tradition there was mudansha all wear white, but I was fairly new to training at the hombu full time, and didn't want any misunderstandings to arise).
So no, I haven't noticed what you speak of.