In Shades of Gray

In shades of gray the values are, the morals and the world. There is not just Black and White, there's neutral tones as well. Reshiram helped shape the laws, the truthful way to live, not unlike the steadiness of fire.
Zekrom helped shape the electrifying desire to progress, to make and create their ideas.
Kyurem, one more Legend of Unova, helped keep the peace between all beings in Unova, unlike His siblings, who fought and bickered.
Two brothers found one Pokemon on their travels, and with the power of this one Pokemon, two brothers governed the nation.
All good things come to an end, however, and soon the brothers fought. True, they had fought before, but this was on a much larger scale.
The Yin in the Pokemon's heart sided with the older brother, seeing the truth in his side of the argument.
The Yang in the Pokemon's mind agreed with the younger brother, seeing his ideads were noble.
The Wugi in the Pokemon's soul could not take the confliction, and in order to bring peace, the Wugi split into three beings.
Reshiram of Truth took on a white coat, white being a mixture of all colors of light. This was to show the certainity of the matter, seeing all points to make one descision.
Zekrom, Being Of Ideals, chose black to show anything can happen, black being an uncertain color.
Kyurem, a balance between his siblings, had grey to show he sided with neither.
Reshiram and Zekrom fought, devastating the land and killing thousands. In the turmoil, they unwittingly killed the two brothers they had sided with.
The three powers soon calmed down, crystalizing their aura into small round gems.
The people of the desert found these gems and worshipped the artifacts, knowing what they were.
Thousands of years later, a boy had green hair found the Dark Stone, and a boy with brown found the Light Stone. These two heros fought for their ideals, for the truth, for the future of the relatonship of mon and man.