I just found out I'm colorblind.

This joke plays on a classic idiom with a clever twist, making it a prime example of wordplay. The humor comes from subverting the common phrase "out of the blue," which means something unexpected or sudden. Here, the speaker, having just discovered they are colorblind, misremembers or misstates the idiom as "out of the purple." This creates an immediate laugh because it's both unexpected and perfectly consistent with the setup – someone who can't see blue might genuinely perceive it as purple, or at least struggle with the conventional color reference.

The real-world context here is the idiom itself. "Out of the blue" has been used for centuries to describe events that occur without any warning, much like a bolt of lightning appearing in a clear, blue sky. Colorblindness, on the other hand, is a condition where individuals have difficulty distinguishing between certain colors, most commonly red and green, but sometimes blue and yellow as well. The joke cleverly merges these two concepts, using the character's newly revealed condition to justify the altered idiom, delivering a punchline that's both silly and surprisingly logical within its own comedic framework. It's a quick hit that relies on a listener's familiarity with common phrases and a basic understanding of visual perception.