Each extreme environment world has its own tech line for habitability (and therefore colonization). The most likely scenario is that you're attempting to colonize toxic when all you've researched is heavy gravity. It's also possible all you've done so far is research extreme colonization, which merely unlocks the appropriate colonization techs, and doesn't actually give you any functional bonus.
Apart from that, you may have somehow modded your race in question to have a negative colonization ability for one or more types of planets (I believe there's a mod in the library that has that as one of its selling points), in which case you'd either need to research the second level of the tech (if you have a -50% natural) or wouldn't be able to colonize at all (if you had a -100% natural).
