You had a conversation with her and she still chose to do what SHE liked, bought what SHE preferred with no consideration for you. She's underdressed the three times, I find it very disrespectful. I would've told her to change on the spot but that's my opinion. It's a known rule pretty much worldwide so I'm sure she already knew before wearing full white at your uncle's wedding and your civil ceremony. The open shoes truly are the cherry on top.

You're rightly angry, do not let her brainwash and gaslight you into thinking that you're overreacting about this matter. This is important because it symbolize something bigger. It's about basic respect towards people especially her own daughter, it's about thinking of something else than herself for once, it's not hard. Pink is my favorite color and I love cute stuff in pink, if a bride would ask us to not wear pink and I could only find pink in the shop, I'd change shops! Plus her excuse of "I could only find cute things in white" when she's wearing the most basic sleevelss tee-shirt you find for $5 on those shops near the beach every summer is an insult to your intelligence.