As a guy, I can't imagine saying that to someone I was married to, and wouldn't have married you in the first place if I constantly thought about darker hair. It seems like a shallow, fickle sentiment, since plenty of women look fine with lighter hair, and personality & character transcend looks over time.

I'd call it a red flag, even if it's been framed as "just being honest," etc. Get him to tell you how important the physical vs. the mental really is. Without knowing subtleties impossible to convey in a text forum, I think your concerns may be valid, especially if he's any sort of smooth-talker or has a profession that requires it. People in sales, for example, can't be fully trusted IMO. Same with those who are big on money this, money that.

Effective liars don't come across as liars, so you have to go with instincts and try to objectively separate him from past instances (the tough part).