I have ready orange, blue and violet icons for all the extreme world types. The idea is as follows:
At violet level, an extreme planet would have a 25% productivity bonus compared to a normal planet. This will require hardcoding but shouldn't be too difficult to implement. Help me here Stardock! I'm willing to borrow and edit the relevant part of the source code and send it in if no one is available to do this.
While we're at it, a few more planet types would be nice, like Pelagic, Volcanic, and Light Gravity. I'm willing to make icons and graphics for these as well.
Finally, I discovered that the ability to colonize extreme planets can be granted by super projects as well as by techs. It doesn't quite display correctly but it does work properly. I'm not sure how that would be fixed though.