bay
騵 centers on a specific horse coat color: a reddish-brown bay horse. The meaning is narrow and concrete, referring to the animal's appearance.
騵 combines 馬 (horse) with 原, which likely contributes the sound and may relate to the idea of a plain or original color, but the exact historical development is uncertain.
A horse (馬) standing on an open plain (原) under the sun, its coat gleaming reddish-brown like the earth: that's a bay horse.
For ゲン, imagine a bay horse named Gen who lives on the plain: Gen -> ゲン, and his coat matches the reddish-brown soil.
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