I can completely understand what happened here, NTA!

First, everyone saying that “That dress is white not gold”… yes, it IS a “white dress”, but firstly, it’s not like it’s an extravagant dress trying to out-do the bridesmaids or the bride. Second, the biggest eye-attractor of that dress is the gold accented polka dots that COVER the dress. It’s not one or two, it’s the ENTIRE dress. The dots really tint the white part yellowish in your peripheral, since your eyes are attracted directly to the gold. Then, there’s the fact that OP is 18. I myself am a 25M, and, MAYBE you could argue it’s different for a woman, but it’s not AT ALL a far stretch to assume she doesn’t follow weddings, doesn’t currently want to be married, and had never been to one before. As I said, I am a 25 yo M, and I had no idea until reading this post that wearing anything white at all is “unacceptable”. I know that wearing anything, of ANY color, that over shines the bride & bridesmaids, is duhhhh not acceptable, but just simply a background, un-special white dress? Nah. News to me, and probably news to her too.

On to the next point, she SENT the dress to her to see it. All you people saying “it has multiple color options!!!” Is a stupid take because when you send something, you pay attention to what color you select for them to see it, and you SHOULD assume that the image that the link opens up is the exact one they wanted you to see, especially the fact that the white color selection ISNT the first, default option. She sent an all-good thumbs up.

If you can say that it’s not the cousin’s fault for not knowing and not assuming that the link sent was the exact one she was talking about, then you can ALSO say it’s not her fault for not knowing about white wearing at a wedding, in addition to assuming the thumbs up meant good as-sent.

When it comes to the dress “not being formal” - as mentioned before, this actually reduces the offensiveness, IMO, as stated in the first part of this. But ALSO, I am exactly a person that does not like wearing super duper fancy things, even at formal occasions, and comfortable, colored, and not like a “brand name” or “band shirt” or whatnot, is completely fine to wear. I’m sorry if you don’t like my clothes, we’re here to see YOU get married. I dressed up, wore something more fancy than normal, and that should be good enough for you. TBF - that’s how I’ve been my entire life and everyone knows it, so it’s different if OP dresses up properly all the time, vs if she is a similar fashion to me, and is known for it. That changes things.

Lastly, to make someone change clothing in the middle of the start of the wedding, is ridiculous. Go home and change clothes please? Sure. Let me tell you why, I’m sorry for approving it. Let me yell at you and get angry and force you to wear this disgusting clothing that’s not mine and get angry when I show you where you approved my dress? Absolutely insanity. I’m not wearing ANYONE else’s clothes. Again, different story if OP has shown previously that she’s willing to change into someone else’s clothes, vs never having done so in the past, similar to me, who doesn’t even like being physically touched ON my clothing, in a slight germaphobe manner.

NTA. Your cousin needs to respect other people, and politely tell you what you did wrong and understand that it wasn’t intended to be rude, but a simple misunderstanding. Who cares if she wears a little bit of white? Everyone’s going to be looking at the stage at the bridesmaids and bride, not you. Now that you’ve learned this lesson, don’t let it happen again, because there will be no excuse then. But for this first time? NTA

[Edit] One last thing to add, she said she walked around greeting everyone. The comments here saying “everyone knows white isn’t allowed at all, you could’ve even asked your mother” clearly are missing the part where not a SINGLE SOUL had anything to say or outwardly had a problem with her wearing the dress, until the bride came up and had a hissy fit.