Consumer surplus before tariff: light blue plus dark blue plus purple
Consumer surplus after tariff: light blue
Producer surplus before tariff: green plus yellow plus red
Producer surplus after tariff: red
Government gain from tariff: dark blue plus green
Why there is a net national gain: the nation consists of consumers plus
government but does not include the producers since they are foreign.
So the dark blue area just passes from consumers to government but is not
lost by the nation. The purple area is lost by the nation, but the
green area is gained. Because in this example the green area
and is bigger than the purple area, there's a net national gain.
Consumers lose the dark blue plus purple areas
Government gains the dark blue plus green areas