I can say that I like a challenge and you tell me it's painless
You don't know what pain is
The “challenge” Kendrick means could be getting through to these empty-souled people and giving them a message. It’s difficult to try to make people listen deep into music these days but Kendrick is willing to try.
The word “painless” holds the meaning here:
Defeating his rivals is a painless exercise because it’s so easy
These rivals are painless — without pain — because they haven’t endured a life as challenging as Kendrick (“you don’t know what pain is”)
In the following, bars, “painless” relates to the metaphor of painting a picture/color blind — i.e. a window that is *pane-less"
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