Yeah but none can say who is pretending to be kind and who is authentically kind.

Right, which is why when people are kind no one is called out for being kind as their "true colors". You're understanding my position well. Its only when deception is revealed that "true colors" phrase is appropriate.

This interpretation doesn’t leave room for “kind” people to ever get angry or have a moment of weakness where they’re unintentionally mean. That’s just a recipe for horrible mental health.

I don't usually associate "true colors" phrase with a moment of weakness. I would think its the more unforgivable things that rank as someone calling out your "true colors". Example: If a person says they treat everyone with respect equally, but then uses racial slurs against people of color then that would be that person showing their "true (racist) colors".

A regular person will never have a racist-moment-of-weakness. A racist hiding their racism will though. That would be their "true colors".