cow or ox of one color, perfect
牷 centers on the idea of a whole, unblemished animal: a cow or ox of a single uniform color, and by extension something perfect or complete.
牷 combines 牜 (cow) with 全 (whole, complete), suggesting an animal that is whole or of one uniform color. The exact historical development is uncertain, but the component relationship is clear.
A cow 牜 that is entirely 全 one color, without spots or blemishes, represents perfection and completeness.
For セン, imagine a perfect, single-colored cow being sold for a thousand 千 (せん) yen: 千 -> セン, and the cow is whole and unblemished.