Warning: Wikipedia contains spoilers
In the novella, real food is too expensive, and the government dispenses rations of food substances made by the Soylent corporation: Soylent Yellow, Soylent Red, and the newest product, Soylent Green.
The latter apparently includes recycled bodies of people murdered by the government for food. (However, this was not in the original novel: Soylent, as the name suggests was derived from Soya and Lentils. Note also that the original book was set in the year 2000, suggesting that some of the projections may have been askew somewhere.)
Soylent Green is also the name of a progressive rock music band.