It depends on the individual really. We went the other way, for years our club had coloured belts for grades, but when we changed associations a couple of years back, the new association only used white for mudansha and black for yudansha, the same as the Aikikai when I was in Japan. Personally, I prefer the white/ black for seniors but I don't think it really matters.
Coincidentally, I have been looking this week at doing something different for my juniors. Currently the association also uses white only for juniors, I believe that in my experience, juniors need a more obviously recognisable form of differentiation and something to progress to. I have stayed away from coloured belts but will introduce stripes on a white belt (1 red stripe = 6th kyu ichi, 2 yellow stripes= 5th kyu ni, 3 orange stripes = 4th kyu san etc.). We have three levels in each kyu for juniors (ichi, ni & san), basically so that a junior will be 29 years old before they reach black belt
. We don't charge juniors or seniors for gradings, and I don't charge for the stripes. Does anyone know a good place to get stripes from, the only ones I have found so far are sold by Century, at an horrendous price of $12.99 for 10?