I don't buy the idea that Disney purposefully painted the icon of their flagship resort colors that look weird in person because they would look good on Instagram. That doesn't pass the sniff test - it's not like the Castle ever looked BAD on Instagram. There's a reason Cinderella Castle regularly ranks as one of the most photographed places on Earth. And even if it didn't look great on Instagram I don't believe they'd really prioritize that over it looking good in person. That's called setting people up for a letdown - hardly the reaction they want Cinderella Castle to inspire as guests turn the corner on Main Street.
I think they made some art, did some tests, and genuinely concluded that the new color scheme was gonna look great in person. But you know what they say - in Theory there is no difference between Theory and Practice, but in Practice there is. I think it's really as simple as "they missed the mark". Surely they didn't spend the money and time to repaint with the intention that it be so divisive, so clearly something went wrong. Even if the Instagrammers love it . . . and not all of them do.
Not to mention that the nature of Instagram Filters is to unify the aesthetic of the image they're laid over, regardless of the subject . . . that's part of why they're so popular, they make pictures look "good" (your mileage may vary) whether they're good pictures or not. There's no need for Disney to undertake a massive repaint to try to calibrate for that.
I think they wanted to try something bold and new for the 50th Anniversary and it just isn't reading like they'd hoped.