It hasn't been in any article in the New York Times since 1981. (I have the search engine.) Goggle asks if I really meant SWIRLED and, and this is the real evidence, it is not found as a found in the Scrabble Dictionary.
Yet, we all all know what it means in the case you've offered: it means colors mixed together incompletely, but then that's what swirled could mean. I believe the writer is being poetic with us, combining swirled and
Whorl together
Joe(I'm twirling, whirling, twisting the night away)Nation