this is not an infinitive. this is a copular verb. that reading would make every verb an infinitive.
the split infinitive rule was made up by Victorian grammarians in an attempt to make English more Latin-like. it has no actual bearing on actual language.
even so, "purple" is an adjective, so the abolition on split infinitives is irrelevant. "purple" accepts adverbs, the reading here being an indication of where red onions stand on the purple scale.
@thedansimonson @Miriamm But "be" is the primary English copular. It's embedded in every infinitive, so daring "it's a cipular verb, not an infinitive" doesn't make sense to me. And the infinitive structure – "is purple" – is right there on the sign. The modifier, "clearly," – again, if you're a semi-purist like me; that's another discussion – therefore doesn't go between "is" and "onion."
Also, my original tweet was a joke.