At our dojo we all wear white belt until we reach shodan. Hakama may be worn at 1st kyu for men, and (until recently) 2nd kyu for ladies. This may seem sexist at first but it is more a statement of the fact that there are few women in our organistation, less even who train to the standard set to achieve 2nd kyu. Another reason for this is that most of the higher grades used as uki for the grading are big guys who really commit to the attack for that level of grading, it's a recognition of the sheer difference in physique of the female taking 2nd kyu and the big hairy guy acting as uki.
I enjoy the fact that i'm a 3rd (soon to be 2nd, fingers crossed) kyu who wears a white belt. In our local classes we all obviously know each others grade, and due to the fact that our organisation has many courses a year you get to know everyone elses grade too. this means that people who enter the class as a beginner don't feel intimidated by seeing several high grades sitting there grinning at them demonically (which we try to keep to a minimum, honestly
It also allows a lower grade to be pushed that little bit further. when i was 5th kyu i was regularly used as uki for the sensei (3rd and 5th dan) at both my local class and on major courses (6th dan and shihan when they visit) and none of them held back much. This was due to the fact that i train with the higher grades as much as i can. however, when i visited a dojo which had a coloured belt system in place i found that the sensei tended to split the class up more, which meant i wasn't pushed as much
In a desperate attempt to get back to some sort of point i'll summarise by saying that as long as you take care when training with any partner you don't need coloured belts to know how to handle uki, and 'handle with care' belts often mean that an uki who is more able than most of their grade isn't pushed as much as they would like