Imagine caring as a value, for example, my care™ about this topic is 2, whereas yours is 5.
In this interpretation, "I don't care" implies that the care™ value of the speaker is 0, by the same logic, saying "I care" implies that their care™ value is greater than 0.
With that in mind, "I could care less" implies there is a care™ value lower than the one they currently hold. Meanwhile, "I couldn't care less" implies the opposite, there is no care™ value lower, which is only true for 0 (AKA "I don't care").
Not really. Yellow paint isn't a thing for shits and giggles, it's there to make the game readable.
Before yellow paint, games needed to have good art direction (instead of "realism") or good environment design to either make it clear something is meant to be interacted with or to point the player in the right direction.
Simply removing yellow paint doesn't suddenly improve art direction or environment design, it just makes the game needlessly hard to read.