This root was the basis for the main Elvish words for “dusk, night”, which was established as Q. lómë in Quenya for most of Tolkien’s life. The earliest form of this root was ᴱ√LOMO in the Qenya Lexicon of the 1910s, unglossed but with various derivatives having to do with “dusk” and “shadow” (QL/55). One notable derivative was ᴱQ. lóme “dusk, gloom, darkness”, which survived in Tolkien’s later writings as “night” and in the 1910s was the basis for ᴱQ. Hisilóme/G. Hithlum “Shadowy Twilights”. Another notable derivative was G. lómin “shady, shadowy, gloomy; gloom(iness)” (GL/45) used in the name G. Dor Lómin, which in the 1910s was translated as “Land of Shadow” (LT1/112).
The “shadow” meaning of this early root seems to have transferred to ᴹ√LUM from The Etymologies of the 1930s, which served as the new basis for N. Hithlum (Ety/LUM), as opposed contemporaneous N. Dor-lómen which was redefined as “Land of Echoes (< ᴹ√LAM via Ilkorin or in later writings, via North Sindarin). The “dusk” sense was transferred to a new root ᴹ√DOM “faint, dim”, which (along with ᴹ√DOƷ) was the basis for the pair words ᴹQ. lóme/N. dû “night” (Ety/DOMO).
These two words for “night” survived in Tolkien’s later writing in both Quenya and Sindarin (Let/308; SA/dú). In notes from the 1940s Tolkien clarified that it “has no evil connotations; it is a word of peace and beauty and has none of the associations of fear or groping that, say, ‘dark’ has for us” (SD/306). The Elves were quite comfortable being under the night sky, dating back to the time when the Elves lived under the stars before the rising of the Sun and the Moon. The root √DOM reappeared in etymologies for star-words from the late 1950s or early 1960s (PE17/152). It appeared again in some very late notes from 1969 where it was glossed “dark” and served as the basis for words meaning “blind” as well as “night”, though this paragraph was rejected (PE22/153, note #50).
Tolkien introduced the root ᴹ√MITH in The Etymologies of the 1930s as a blending of ᴹ√MIS “✱wet” and ᴹ√KHITH “mist, fog”, with the derivative N. mith “white fog, wet mist” as in N. Mithrim “✱Mist Lake” (Ety/MITH, RINGI; EtyAC/MITH). As a later addition to this entry Tolkien wrote the adjective N. mith “grey”, and that was the more common use of this word in Tolkien’s later writings. In a 1955 letter to David Masson Tolkien specified that “usage suggests that MIÞ- is paler and whiter, a luminous grey” (PE17/72).