Why is 生地黃 called 地黃 [yellow earth]?
黃 is sometimes a plant name: 大黃 'rhubarb' (this used to be dai-huang, not da-huang), 麻黃 ephedra'; 地 sometimes means a tuber, seems that's not the case here. One source identifies 3 varieties: 天黃, 人黃, 地黃.  This is the 三合 system.