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
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.