Check for the file Colorset_Default.zip in your Add-Ons folder. If it's there, delete it. Then copy the one from inside System_ReturnToBlockland.zip to your Add-Ons folder, and restart the game.
Re-install the game
I have the same problem. But there is no Colorset_Default.zip in my System_ReturnToBlochekland.zip, only the Default_Colorset.txt. Moving it to my Add-Ons folder did not fix anything.
Put it in a folder called Colorset_Default I guess, and throw a description.txt in there too. RTB should be creating one for you. Make sure your Blockland folder isn't in Program Files or something dodgy like that.