Nice WIP, curious to see what it's gonna become.
I was wondering if it's possible to have an unsolvable configuration, and as long as there's 4 of each color and an empty flask, it's always solvable :
1 is flask1, 2 is flask2 and E is empty flask:

You can swap two top slices of flasks 1 and 2 :
1->E 2->1 E->2

You can rotate the content of a flask (1):
2->E 1->E 1->E 1->E 1->2 E->1 E->1 E->1 2->1 E->2
That's enough to get every possible order.(albeit not very fast)

The puzzle should always be solvable. The current level gen simply creates 7 flasks filled with 4 of the same color. Then it shuffles layers between flasks, followed by shuffling the layers within the flasks, and leaving the last flask empty.