I feel for you … embarrassing indeed .
Perhaps convince him to put a temporary henna on his skin as a compromise and see if he gets sick of it ?
Only the decide for the real thing .
Your issue is, the more you protest the more he will want to “be a free spirit” and imagine himself like a young man with no nagging from his wife

Men can be so ridiculous.
Perhaps if this is as bad as it gets for his midlife crisis count your blessings.