I believe I've found a minor bug in the colony cost formula. I found a moon of Sirius B I with the following atmospheric composition.
Ammonia (F) 82%
Nitrogen 17. 6%
Neon 0. 36%
Total Pressure is . 1
Gravity is . 73
Temperature is -42. 8
Using the default human race.
Anyway, the problem is that although the moon has an unbreathable atmosphere (which is indicated on the system view), the colony cost is 1. 90, which is equal to the temperature cost, when it should be 2. 0 on account of the unbreathable atmosphere; there isn't any oxygen. If the ammonia was gaseous, the colony cost would also be set to 2 because of the toxicity. I'm guessing the program is disregarding the toxic gas since it's frozen (as it should), but then doesn't do a follow up check for oxygen content, so it just becomes the temperature cost.
I've got a database to send if you need it.