chinaberry tree, round, harmonious
欒 centers on the chinaberry tree, and by extension the round, harmonious clusters of its flowers or fruits. The abstract senses of roundness and harmony are natural applications of that tree's appearance.
欒 combines 木 (tree) with the phonetic component 䜌, which likely contributed the sound and may have originally depicted something round or clustered. The exact historical development of 䜌 is uncertain.
The top part 䜌 looks like a tangled cluster of threads (言, 糸, 幺), and the bottom is a tree 木. Picture a chinaberry tree with round, tangled clusters of golden berries hanging from its branches.
For ラン, imagine a chinaberry tree running along a riverbank: run -> ラン, and the tree's round clusters sway harmoniously.