There are four balls of the same size and weight, red, yellow, blue and green. Take a scale. If you put the red and yellow balls on one end of the balance, and put the blue and green balls on the other end of the balance, the  scale will remain balanced. If you put the red and green balls on the left side of the scale, and the yellow and blue balls on the right side, the left side sinks. If you put a red ball and a blue ball on the left and a yellow ball on the right, the right side sinks.
Blue, red, yellow, green is in light-to-heavy order.