What a boar! A combination of all of the techniques sounds like the ideal way to go.
Start by ignoring him, until he repeats himself to you. Then say "I'm sorry, I missed that", so he has to repeat it a third time "My wife's hair is natural not like yours!". Look blank for a second, them just say ok and turn back to whoever you were talking to.
He will up the ante and become more and more obnoxious. Continue the faintly bemused waste-of-time act. When necessary, pull in your husband or someone else, restating Boar's words with the correct inflection "he thinks I bleach my hair, honey". And the final one, which I love, is "Oh bless your heart, you think your wife is really special, that is so cute".
Have a friend primed to catch your eye and stifle giggles together too. By the end of the meal he will be fuming but you haven't been rude and you don't have to see him for another 4 weeks - when, sadly, you'll probably have to repeat this all over again until he gets the message.