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).
The Elvish words for “blue” remained very similar throughout Tolkien’s life, but underwent a number of minor conceptual shifts. The word ᴱQ. lūne “blue, deep blue” appeared in the Qenya Lexicon of the 1910s in a collection of words along with ᴱQ. lūle “blue stone, sapphire”, but no root was given (QL/55). The word for “blue” in the contemporaneous Gnomish Lexicon was G. luim (GL/55). In The Etymologies of the 1930s, the root for “blue” was ᴹ√LUG with derivatives ᴹQ. lúne and N. lhûn (Ety/LUG²).
Meanwhile, the root ᴹ√LUY appeared in The Etymologies with derivatives ᴹQ. luina and Dor. luin “pale” (EtyAC/LUY), probably connected to ᴱN. Draugluin “Werewolf Pale” from the Lays of Beleriand of the 1920s (LB/205). But in The Etymologies the root ᴹ√LUY was rejected, and Dor. luin “pale” was reassigned to ᴹ√LUG² and then revised in form to Dor. lūn (Ety/LUG²; EtyAC/LUG²).
In addition, there was already evidence of a conceptual shift in the Noldorin words for blue in the 1930s, with the name N. Eredluin “Blue Mountains” being given as an alternative to N. Lhúnorodrim and N. Lhúndirien “Blue Towers” (Ety/LUG²), the latter appearing as N. Luindirien in contemporaneous Silmarillion narratives (LR/267). By the 1950s and 60s, the Sindarin and Quenya words for “blue” had firmly become S. luin (Let/448; S/54; UT/390) and Q. luinë (LotR/377; PE17/66, 71). The root √LUY “blue” appeared in notes from the late 1960s serving as the new basis for these “blue” words (VT48/23-24, 26).
All this made a mess for the river name S. Lhûn (LotR/1134) from The Lord of the Rings which was a remnant of Tolkien’s earlier ideas, and he struggled to find a new basis for that name as discussed by Patrick Wynne in his article on The Problem of Lhûn (VT48/26-29).