rainbow
蝃 centers on the rainbow, a colorful arc in the sky. The given meaning is a single concrete image, so the core meaning stays close to that.
蝃 combines 虫 (insect) with 叕, which likely contributes the sound. The exact historical reason for the insect radical in a rainbow character is uncertain.
The left side 虫 suggests a creature, and the right side 叕 looks like a series of connected arcs. Picture a colorful insect-like arc stretching across the sky: 蝃 is a rainbow.
For テイ, imagine a rainbow shaped like the letter T, and you say 'T, eh?' as you point at it: T-eh -> テイ.