You can make six 2 with red at the top because you can switch blue and yellow 2 with blue at the top for the same reason and 2 with yellow at the top for the same reason. If you add a green block it makes it 24 because if you have green at the top and then red underneath the green you can have yellow at the bottom and blue above yellow and then if you keep the green and red in the same spot but switch the yellow and blue that makes a second tower. Then if you keep green at the top and put red on the bottom then you can put blue on top of the red and the yellow on top of the blue the switch the blue and yellow and then thats 4 towers. Then if you put yellow on the bottom red above the yellow blue above the red and green at the top then switch around the blue and yellow thats another 2 towers so you now have 6 and then if redo the whole process but switch where the green and red went thats another 6, and then do the same as the first on but swap blue and green for the whole thing thats another 6 and then do the first one but swap the yellow and green for the whole thing thats another 6 and 6 + 6 + 6 + 6 = 24