This is entire summoning process is one big reference to Final Fantasy Unlimited, in which one of the main characters has an arm cannon that can summon Final Fantasy's famous espers/eidolons/GFs/whatever. He does so by putting three special colored bullets in the cannon and then firing. Each bullet had some kind of poetic description and its name always consisted of *element/emotion/abstract concept/whatever* + *color*.
And like the Soul Gems, these bullets are made from people('s souls).
This is entire summoning process is one big reference to Final Fantasy Unlimited, in which one of the main characters has an arm cannon that can summon Final Fantasy's famous espers/eidolons/GFs/whatever. He does so by putting three special colored bullets in the cannon and then firing. Each bullet had some kind of poetic description and its name always consisted of *element/emotion/abstract concept/whatever* + *color*.
And like the Soul Gems, these bullets are made from people('s souls).