Joringel has those berries. They would make great purple dye.
How to get them?
He can't do it
The berries are free
He is interested in Jorinde. Talk to both of them. First Jorinde and then tell Joringel someone cares about him.
If he would show her some token of affection. Like a flower that tells it. Read about it from the biology book.
Get the flower from Jacob. Give it to Joringel.
Take the berries from the lunchbox. Give them to Hans.