Do you only use/have 400 Dice chips at a game now or do you end up adding more chips due to re-buys?
It would help to know your blind levels, lengths and if you color up at all but here's my thoughts.
Assuming levels as follows:
25-50
50-100
100-200 (Color up green)
200-400
etc...
(Depending on your blind levels) Lets assume you will have your max number of players (

using your current breakdown. You won't have enough green or black. For re-buys or add-ons you would need to use larger denoms and there would be a lot of change making. Even if you have only 6 players this may/will happen. Some don't mind this, some do.
You could easily cut back on the purple and yellow (maybe even eliminate the yellows) and bump up the green and black.
I have a 1000 pc set of Bees, run T1500 games, 20 min rounds, 10-20 players, and the games rarley go long enough (3 hours or so) to even use the yellow chips. When I do it's usually at the heads up stage anyway. I don't even use many purples for that matter.
Since you only need green for 2 rounds I'd probably go something like this...
150 - green
175 - black
50 - purple
25 - yellow (if any, otherwise I'd do 200 black)
Just my .02
Ranman