Am I being unreasonable to interfere with brides choice?

I don't think you were unreasonable at all. There would be so much gossip, shock and horror at a "white English" wedding if a load of guests turned up in white bridal gowns. It is the kind of thing you might see in a comedy movie.
If I was going to a wedding of a different culture I would appreciate the assistance. I imagine your bride has probably rejected all assistance from the groom's family and called them interfering.

Agree that she was defensive because she was embarrassed. There is also another term for when you pick and choose bits of another culture without making the effort to understand and be respectful. Lots of women dressing up as Indian brides for a friend's wedding, not listening when it is pointed out to them that this isn't appropriate, is textbook cultural appropriation.

You were very rude. It’s nothing to do with you. It’s between the bride & the groom- he should be the one to advise her or not.

All the Asian guests will be laughing at this English bride and her friends and family who are on an exotic dress fetish. It will be very clear to all the Asian guests that this bride doesn't have any respect for the culture that she's marrying into. She is just there for the dressing up opportunities and isn't interested in actually learning about the culture.

Not a good start to a her marriage imo, it's also very telling that the bride didn't invite any member of her fiancé's family. She should be asking advice about bare skin especially if the community is a bit conservative. In my community, both mums would attend as the groom's side would pay for the bridal outfit.

I agree - this would be my reading too. She may also not have understood the distinction you were trying to explain.

I would have been embarrassed in her shoes but ultimately grateful that somewhat warned me. It's horrible when people silently look on and watch you struggle or make a mistake instead of helping.

I also think some posters are being either naive or disingenuous to suggest that if an entire bridal party turned up to a church wedding in white bridal gowns that it wouldn't become a source of gossip and jokes. Some people might be nice about it but plenty would leak it out into the wider community and take the piss or be judgemental.

In 6 months there will be an aibu about the bridesmaids being derided at an Indian wedding for all turning up in bridal outfits and why didn't the shopkeeper say something?

I think you did the right thing, and at least you tried.

The problem is that I think a lot of white people (I’m white btw) see Asian formal wear and think, ooh, bright colours, gold, sparkles! And they think it’s all the same because it’s all bright and pretty and shiny, without appreciating the hugely significant cultural nuances.

If at an English wedding, the bride wanted all her friends and family to also wear wedding dresses, isn't that her right? It's their wedding, and maybe that's what they wanted.

Perhaps you spoke to them in a condescending way and she became overly defensive because of that?

Perhaps the bride-to-be genuinely doesn't want to follow convention and wants to follow her own path?

Perhaps she doesn't care about other people's opinion including yours or extended family?

It's hard to know how someone is going to react when a random stranger gives unsolicited advice. The bride was defensive but maybe she felt entitled to be defensive when you interrupted what was going on?

I would want to be told, if it was me. Obviously if I'm not brought up in Indian culture, I'm going to have gaps in my knowledge and would be glad someone saved me from accidentally dressing my entire family in Indian bridal wear. I suppose maybe it depends how you said it but I think you were trying to help.

In your situation I would have just apologized, explained my intention and left it at that. I don't think you did anything wrong on the whole, it was obviously just wrong for them. They might even go home and reflect on what you've said anyway and take your advice. Maybe they felt silly at their mistake and were trying to save face.

You were lovely OP. Some people are just ungrateful twats.

I think you did the right thing. It’s not surprising that she reacted defensively I’m afraid, people do, but I bet as a pp said she will think it over afterwards and maybe check with her mother in law to be/ future SILs (or more likely remember the advice that her future MIL has been desperately trying to give her!!) So I think you will have helped. Ultimately

It is true that customs change as cultures meet each other but this certainly sounds like a big cultural chunk of knowledge that she needed even if she didn’t really want it.

Oh and can you name and shame the shop please so people can avoid it!

I would have thought she'd change her mind pretty sharpish when she saw the prices of those outfits 😂

The shop keeper was a total cretin.

You’ve done all you could, OP. Looks like she’ll have to learn an embarrassing, expensive mistake on her own.

It’s also a little bit of the bride’s fault for not bringing someone from her fiancé’s culture to help her understand what she was choosing.

But what culture would they be crossing? I do t know any culture where the wedding party all arrive in bridal attire, which is what the OP is talking about. I’d have been very appreciative of the advice @Allequal because I clearly in that context wouldn’t have understood the culture and wouldn’t want to make a faux pas on my wedding day.

10 years deep I am into a mixed marriage, Pakistani/Muslim family. I totally understand why you did what you did and think it was very kind of you to try and help her out. More fool her for not listening to you. She’ll soon understand how the aunties gossip and stuff. You really tried to save them from being ripped off and being embarrassed etc. it’s so kind you tried. Need more people like you x

You were completely right and being helpful. Not sure why others on here would disagree. 🤔

I think offering well-intentioned insight into your culture is absolutely reasonable, OP!

I think she was on the defensive because she was embarrassed. First , for not knowing the difference between pretty outfit and bridal outfit , and second the possibility of being taken advantage of by the shop owner. It was easier to take it out on you/distrust you.

You tried op

shame the bride to be would rather remain ignorant or maybe she will take on board your conversation

if not it sounds like a scene from a comedy film about inter racial relationships

YADNBU. How awful a faux pas to have all the women turn up in wedding dresses-the shopkeeper must have been rubbing his hands in glee. I would’ve been grateful at your intervention and help.

The only thing that makes me doubt if it was the right thing to do, is that if I were the bride I'd probably have googled "what should bridal party wear at Asian wedding" or similar.

But otherwise, of course pointing out the problem they were generating for themselves was kind.

Have you never wanted to be helpful to anyone?
Edit - that was to @PollyBell

I think you should have included the info about the shop owner and his urdu conversation in your opening posy as it would probably have given a different spin.

Ultimately you tried to educate her. You can't force her to listen though.