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I'm not completely sure because like Eginhard38, I was doing it by trial and error too, but I wrote this on a file when I did the changes:
  • First Line - Province Unselected
  • Second Line - Selected
  • Third Line - Border(Color between the province border and the province color)
I never changed the index because none of the other existing mods made any changes, so I have no idea what does it do.

The water is completely different to the regular colors, and i'm afraid that I really never quite understood what I did, however, if you want to use this notes as a guide go ahead.
  • zoom out water
  • water deep
  • shore-font
  • province border-font
This is probably innacurate because I remember that after writing this I noticed that some changes that I made based on the above were wrong, but you can use it to try and find out what line does what.
 
hehe, I was doing it with Trial & error too, but with nearly no result...
seems I have not enough endurance ;)

But your notes are something to start, very helpfull!