<<The use of ginger in the US to describe red hair took off with publication of the first Harry Potter book in 1998, says Kory Stamper of Merriam-Webster. Unlike in the UK, there is no anti-ginger prejudice in the US, she says - Americans think of warm, comforting things like gingerbread.>> http://www.bbc.co.uk/news/magazine-19670686

So absolutely no slur was intended. In U.S. popular culture there are some (mostly tongue-in-cheek) stereotypes like fun dumb blondes, smart passionate brunettes, and sexy hot-tempered redheads, but no particular prejudice against any of them, and I was just laughing at a line I had seen somewhere.

In any case, I apologize for any unintentional cruelty to red-heads.

On a final note, anyone dying his or her hair in the U.S. thereby takes on the characteristics of that hair color, natural or not, and is fair game for those jokes. This is true regardless of ethnicity. :_)