Most of the purple pillars are hints to figure out what goes in the empty purple pillar east of Sablee, not too far from the beach.
You got it right for the sandcastle, but the hitbox for where to drop the sand is a bit wonky, that's something I need to fix. You have to drop it precisely on a part that's already built, the middle won't work. Sorry about that...
Now that you have the emblem you'll be able to craft, you just need to talk to Eveii, one room to the right of the big red crystal in Sablee. She has "alchemy" written over her head ^^
There's also a walkthrough here: https://www.patreon.com/posts/walkthrough-by-33962675
It should have everything you need ^^