Smoked herring is not, as the idiom will claim, 'red'. It is more of an orange, coppery colour. That means the term 'red herring' is something of a red herring. Which tracks.
The process of smoking a herring has changed over the years. In the Second Age, a herring would be smoked in the village elder's pipe, in the hope that some of the elder's wisdom would transfer to the fish. In the Third Age, entrepreneurial Zamorakians would line their demon portals with herring in the hope of an infernal aftertaste. Nowadays, people use a cooking pot.