Country_guy14 starts with the most familiar rhyme in English and immediately picks a fight with it. Roses are red, sure, but violets aren't blue, they're violet. The speaker keeps going, piling up more examples of language that doesn't match the world: apples are red too, an orange is orange (at least that one behaves), Greenland isn't green, and pinkies aren't pink. The question "what does it mean?" sounds half serious and half amused, like someone who has just noticed a strange fact and can't let it go.
The mood is playful and a little mock-indignant. The line saying that calling a violet blue "defiles it" is deliberately over the top, as if color accuracy were a matter of honor. Then the last line deflates the whole rant with a shrug: it's just hard to rhyme "violet." That confession is the joke, and it also hints at a real point, which is that sound often wins over truth when we write verse.