We aren't conservative in a political sense at all, but Christian backgrounds
Conservative Christian backgrounds. Is it non-christian to wear nail varnish? Plenty of Christians do, women or men.
Frankly? I nave never bought into gender roles, ever since i was a very small child. My dad was what people might think of as a manly man with a very manly man job. But he and my mother brought us up to do the things we liked and wanted to do, especially encouraged us at things we were good at, and as a result i ended up in a pretty manly man job as a woman, as did my brother. It never stopped me wearing nail varnish, combat boots, high heels, dresses, doing woodwork as a hobby whatever whatever.
If that is how your husband is looking at it: "i fancy a bit of colour on my nails, it looks interesting" but otherwise nothing else, then being confrontational will just annoy both of you.
If your husband is starting to explore something that he has kept hidden for years, confrontation will drive it into the shadows. What you need is some open communication about it. So that you can both move on with your lives, together or separate, but understanding each other better.
Are you posters seriously saying that if your husband started to wear say cerise pink or mauve nail varnish on all ten nails in his everyday life, you wouldn't mind??
Cerise would look great with my husband's colouring. He also wears baby-pink shirts sometimes because they make his skin tone look nice.
If he did start dressing or behaving very differently all of a sudden i would wonder why. Affair? Trans? Gay? Just fancied it because we're fairly open about things like this in our house. But i would just bring it up over dinner, or a cup of tea and i definitely wouldn't be confrontational.
And then I'd see how i felt about his answer and take it from there. I have been married for a long long time though, if it had only been a few years and/or i had young children i would probably have different reactions.