Extreme planets are red in the galaxy map to show they can't be settled until the Extreme Colonization tech is researched. I had a game where they stayed red after I got that tech. I tried to repeat the problem many times and failed every time. After thinking about it, I remembered that I got the tech in a trade instead of researching it. I was able to do more testing and verify that was the cause. I have a save available here that shows the problem in action.
Load the save "Extreme Planet Color". Zoom out and notice all the red planets. Check them and see that some require extreme colonization in order to colonize them.
Others require different techs. Go to next turn.
Acknowledge completion of research for Universal Translator, and choose any new tech to research. Acknowledge the event about completing the research.
Contact the Yor, and propose a trade. Ask for their Extreme Colonization tech, and offer both of your techs, open borders and enough credits to get them to agree. Accept the trade, and leave the trade screen.
Look at all the red planets and see that all are still red. Check closely and see that the extreme planets are still red but no longer require the extreme colonization tech since you now have it.
You can keep playing if you want to. The extreme planets will stay red.
However, saving the game and loading the save will fix the problem.
This problem only happens when the tech is acquired by trade. I have verified that if it is acquired by research, the planets change color correctly. I don't have a save that shows that. I have no idea what happens if the tech is acquired by a race gifting it to you.