red, vermilion, name of ancient
彤 centers on a vivid red or vermilion color, and is also used as an ancient name. The color sense is the primary conceptual core, with the name usage being a specific historical application.
彤 combines 丹 (cinnabar, red) with 彡 (hair, pattern), likely reinforcing the idea of a red color or decorative red pattern. The exact historical development is uncertain.
The left side 丹 is a red mineral (cinnabar), and the right side 彡 looks like decorative streaks. Together they paint a picture of bright red vermilion streaks.
For トウ, imagine a red toe painted with vermilion: toe -> トウ, and the red color matches 彤.