gland
脃 centers on a gland, a bodily organ that secretes substances. The meaning is specific and anatomical.
脃 combines ⺼ (flesh, body part) with 色, which likely contributes a phonetic element. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a body part.
The flesh radical ⺼ on the left indicates a body part, and 色 on the right can be thought of as a colorful secretion. Imagine a gland producing a colorful fluid.
For セイ, picture a gland secreting a 'say' (セイ) colored liquid, as if it's speaking through its secretion: say -> セイ.