Yeah sorry, I've been meaning to put out a post-gamejam version that has a clear narrative as well as button prompts but stuff kept getting in the way.
Once you crushed down the petals into pigments and have lead ingots, you should be able to press F next to the alchemy bench (bottom left of screenshot) to interact with it which lets you paint transmute the ingots.
Thanks for playing the game! <3
There's another game jam by the same community next month and I'm hoping to try that too.
EDIT: Just tried it, it's F next to the bench, not Q