There are 8 possible colors for any slot. These are (1) red, (2) orange, (3) yellow, (4) green, (5) blue, (6) violet, (7) white, (8) black.

When you insert a piece of glass into a slot, you add that color (number) to the slot color. When a slot reaches black, the slot to the left is also advanced by 1, so you should start solving from the last slot.

* Slot is (7) white, you add (4) green, it will reach (3) yellow and change the slot to the left by 1.

* Slot is (5) blue, you add (8) black, it will reach (5) blue and change the slot to the left by 1.