The 丹 kanji– WaniKani / Kanji / 丹 –uses “rust colored” as its definition, but then all the vocabulary that build on it seem to be using the alternative “diligent” meaning. Doing a quick, unprofessional search, it feels like the on-yomi is more often associated with “diligent”, while the kun-yomi is more often associated with the “rust colored” meaning.
Suggestion: Include both meanings on the kanji, and maybe word the mnemonic to be about diligently maintaining a rusty old broken boat just to keep it afloat. Would also be good to include a vocabulary which uses the rust colored meaning to help solidify that.