丹(dān) - cinnabar red, elixir pill in Mandarin

丹 (dān) started as cinnabar, the red mineral ground up for pigment, and it still carries that deep vermilion. From the same ore came the pills of Chinese alchemy, so 丹 also names a prepared pellet of medicine: 丹药, 仙丹, 炼丹.

In modern Mandarin it almost never stands alone. You meet it inside 牡丹, the peony, inside old medicine names, and above all inside personal names, where it is one of the commonest single characters going. 丹心, a cinnabar heart, is the classical way of saying unwavering loyalty.

The trap is the sound. 丹 dān, 单 dān (single) and 但 dàn (but) crowd the same syllable, and 但 is the one you will actually need in a sentence. Learners also assume a name syllable is just a noise: it is not, and knowing that 丹 is a colour tells you something about the name.