yellow, augment, increase
黈 centers on the color yellow and the idea of adding or increasing something, possibly through a yellowish tint or augmentation.
黈 combines 黄 (yellow) with 主, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with yellow and the concept of augmentation.
The left side 黄 means yellow, and the right side 主 can suggest a master or main thing. Imagine a master painter adding more yellow to a canvas, augmenting the color until it dominates.
For トウ, picture a yellow toe being augmented with a golden ring: toe -> トウ, and the yellow toe gets an increase in decoration.