What follows is an e-mail I sent to Pompom which got no answer.
Then I found this board and since the best players are here, I'll ask away:

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Yesterday I finished the game on white belt, and the game started over at a
new difficulty (I'm guessing yellow?)
The fact is that the multiplier carried over...
The point is: Is the best strategy to obtain a good score starting on white
belt and then trying to complete the game at each subsequent difficulty?

The point I'm making, is that if you take a very good player (e.g. someone
who can finish the game at belt 4 without game over) why should he NOT start
at white belt and work his multiplier up during that belt to score much more
in the subsequent ones?

But this would make the belt selection almost pointless (?)

I'm just trying to understand if there are some big bonuses to starting,
say, on yellow belt, rather than starting on white belt, assuming I can
finish white belt without game over, and thus carrying over to yellow belt a
multiplier of 20x and more.

Hope what I'm asking it's clear.

But if you start on Black belt your scoring, essentially goes  10x 20x 30x etc etc.  starting at white it goes 1x 2x 3 x etc

Yes you CAN loop through from white belt onwards but, from what I know, it loops through on black as well. So if you can handle black belt and beat the game you get another go at the easy levels with INSANE multpliers possible.

Once you can easily beat the game on white belt go for yellow you'll get 2 x your score and more after a little practice.

I got my 375 million high score by starting on Yellow Belt. The exponential on the multiplier is fantastic. You get 2x the points starting on Yellow Belt that you do on White Belt.

After three rounds through, you can get millions of points on each level if you work them correctly. Beating each level with shields intact also gives you tons of points. You shouldn't do this on the levels where you can wrack up millions, but on some of them it pays to keep that shield intact. And, of course, you don't lose your multiplier.

In the end, a multiplier of x50 on Yellow Belt is BETTER than a x90 on White belt.