bitter orange
橙 centers on a specific type of citrus: the bitter orange. Its meaning is narrow and concrete, referring to the fruit or the tree.
橙 combines 木 (tree) with 登, which likely contributes the sound トウ. It originally referred to the bitter orange tree.
A tree 木 that produces a fruit you might climb up to pick, like climbing 登 a ladder to reach bitter oranges.
For トウ, imagine a bitter orange so sour it makes your toes curl: toe -> トウ, and the tree 木 with the climbing 登 shape is where you found it.
bitter orange (Citrus aurantium); sour orange; Seville orange
orange (color, colour)
natsumikan (Citrus natsudaidai); Japanese summer orange
amanatsu (Citrus natsudaidai); Japanese summer orange; less sour variety of the natsumikan
orange-peel oil
neroli oil (essential oil made from bitter orange flowers)
orange-red; reddish orange; fire red
bitter orange juice (used as a food acid)
orange (colour)
pale orange (color)