Head to the House of the Hero at (G-3). Check the front door. You will receive a cutscene where Carbuncle will ask you to obtain all seven "colors of light." The minister Ajido-Marujido will then find you and speak to you about him. (Note: It is possible that it will be the Joker Cardian that comes instead of Ajido-Marujido. The Joker will come in the beginning of the cutscene and tell you to listen to the ruby. You will be told to gather the colors of light and the cutscene will end.)
The seven "colors of light" that Carbuncle requires correspond to the colors of a rainbow. They are obtained by zoning into an outdoor, enemy-populated area while one of six weather effects is active—the seventh is obtained by zoning into an outdoor, enemy-populated area with no weather effect at all.
To clarify: "Outdoor" means that you may not zone into a cave or dungeon area to obtain a color.
To clarify: "Enemy-Populated" means that you may not zone into a town or city to obtain a color.
Zoning into an area with the above requirements (that you have not yet obtained) and with the ruby in your inventory will trigger a short cutscene upon entering the area, where Carbuncle will remark on the color of light that you have found. (You will not see the same cutscene again.)
This can be done by using an outpost teleport into the zone.
This can be done by teleporting into the zone (Teleport-Dem, etc).
Weather CheckerNPCs in each town or city can help predict when and where the weather you will be looking for will be in effect. Bear in mind, however, that these Weather Checkers will not be completely accurate; some weather effects may occur on days they are not predicted to be on, and vice-versa.
Locations of areas meeting the requirements for Carbuncle's "colors of light" are as follows:
"Double" Weather Effects in those zones will also count toward receiving a color.
Note that Light and Darkness weather are not included in this quest. Carbuncle will not react when zoning into areas with these weather types. Also note, wind and thunder may appear in Lufaise Meadows or Misareaux Coast but the ruby will not respond to them. This is due to the fact that the Summoner job was added in with the Rise of the Zilart Expansion. These areas didn't exist at that time.
When all seven colors are obtained, you will receive an additional cutscene where Carbuncle will fly off to La Theine Plateau. Head there and go to the stone circle at (G-6).
Trade Carbuncle's Ruby to the ??? at the center of the circle. You will receive a final cutscene, after which the Summonerjob will become available to you.
It is worth noting that, for each time that a player completes this quest, a rainbow (the same one as seen in the cutscene) will appear over the Crag of Holla on the next Vana'diel Day (so long as the weather is not Rain). Should it be rainy weather, the rainbow should appear shortly after the rainy weather stops if it is before 18:00 in-game.