The entire Emotional Spectrum was open to the Guardians of the Universe from the beginning; The Guardians used the Manhunters to police the whole spectrum, but the Manhunters went overboard and decided the only way to control the emotional spectrum was to do away with the source; sentient life. This didn't go over well with the Guardians so they destroyed/shut down/boxed up the Manhunters then tried again with the Lanterns.

Each colour of the emotional spectrum has a light and dark side

Red - Passion/Courage and Rage/Hate

Orange - Protection and Greed

Yellow - Temperance/Caution and Fear

Green - Tranquility/Peace and Emotionless/Numbness

Blue - Hope/Faith and Submission

Indigo - Sacrifice and Hopelessness

Violet - Love and Obsession

The spectrum is book-ended with the powers of creation in the form of the Starheart and destruction as the Heart of Darkness.

To insulate their operatives from the raw energies of the spectrum - and the corrupting influence of the dark side of the spectrum - the Guardians provided rings and lanterns to act as circuit-breakers. While it is easier to draw from the dark side of the spectrum, and the user can draw more heavily making them more powerful on average, the wielder also burns out quickly, even risking self-immolation.

The Guardians divided the universe into 3600 sectors and in each sector was established a Lantern Station House. Two members of each Corps, a primary and a secondary, were assigned with a sector chief to a total of fifteen lanterns per sector. In sectors with untold billions of inhabitants that's not many.