This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
栬 combines 木 (tree) with 色 (color), but the historical origin and original meaning are unclear. It may be a phono-semantic compound, but the exact development is uncertain.
A tree 木 with a colorful 色 appearance, but its meaning is uncertain.
For セイ, imagine a tree that says its own name: 'Say' (セイ) the tree, though its meaning remains a mystery.