Hello. A bug looks to be preventing the RectangleFontColor skinparam working on Activity diagrams (the new syntax version).
For an example, please see http://www.plantuml.com/plantuml/png/RSz12i9030NGVKuHUW7tjPIgu06ymT36CZXDIkOhYdXtxO2Yu2QB_4S-wJA2upmbogUeS_0myK5w11sJRCq7yPqbSnvTHF-XlIc-2PMBvE-Sgm-j-4xCzQP7l4JSrlIWOxA0g2EhGUeV2xc2OruIRlgbeIuDRPdDggnQoh2PEj7XzSGJ
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Hello A.,
For that, you can use:
skinparam PartitionFontColor green
Then we observe the expected result:
If that can help,Regards,Th.