What you are trying to do is not actually possible with the current version of the Treemap. This is due to the fact that the ColorEncoder overrides colors in the treemap nodes at certain points when the data or colors could have been updated.
However, we recently built a version of the treemap for a client that adds the ability to adjust the properties of nodes (including color) automatically based upon certain criteria that can be passed by means of an annonymous function. This code would probably solve your dilemma. I will try to get it pushed out into the unstable branch as soon as possible.
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