The system is easy. Just drink milk from the jar with the color from the earliest day in the week you can find. The color label is a circular sticker placed on the lid and sealed with a small piece of shipping tape to make it a bit more waterproof so it will last longer.
The daily totals of milk are recorded on a monthly chart based on a four week system on the refrigerator. The day of the week is colored as a reminder. Daily totals are tabulated at the end of the week and end of the month and recorded on a separate yearly chart. I took this picture a couple weeks ago and we just finished this chart. We got over 61 gallons of milk in September!!! (A third of which went down the drain in botched mozzarella recipes.) I also measure in gallons instead of pounds like I should. I would forever be trying to calculate and convert it into the gallons that I can understand.