Soylent Green/WMG

If Soylent Green was produced in ancient times, the famous scene would've gone like this:
"This is madness!"

"Madness..."

"THIS! IS! PEOPLE!!!"

Soylent Orange is organic garbage!
Dug out from the landfills and sewers and broken down into an edible format. They're not wanting for energy, and it's not like they care any more about the environment, they just need food without wasting space. Soylent Red, on the other hand, is exactly what they say it is, made from the plants available from the few remaining hydrobotanical gardens underground.

People were already being farmed...
Consider the terrible state of the cities, the encouragement for suicide and the money for corpses. Consider that people cannot get out of the cities. It's not illogical to presume that people are already being farmed as food; they just don't know it.

The whole movie was a Xanatos Gambit by the government to make people lose their hunger for Soylent Green
It actually is just soy and lentils, but the demand for it is so huge that the government developed an extremely intricate series of events to have somebody discover their false planted evidence that Soylent Green is people, then let him escape and reveal it to the public. That way, the government won't have to keep producing this expensive product.

Thorn was taken away to be killed.
The Company now knows that Thorn knows too much. The people hauling him away on the stretcher are employees of the Soylent corporation.

Soylent Red and Yellow are also people.
Soylent Green just tastes better due to technological improvements.