A root whose most notable derivatives are Q. alqua, S. alph “swan”. The earliest iteration of this root was ᴱ√ḶKḶ from the Qenya Lexicon of the 1910s (QL/30); the other derivatives of this root from this period have to do with “appearance” such as ᴱQ. ilk- “to seem” (QL/42). By The Etymologies of the 1930s this root took on its later form, ᴹ√ALAK and had the gloss “rushing” with other derivatives like ᴹQ. alako “rush, rushing flight, wild wind”, N. alag “rushing, impetuous” and N. alagos “storm (of wind)” (Ety/ÁLAK). It was also an element in the name of S./N. Ancalagon “Biting Storm”. Given the continued appearance of this name of The Silmarillion (S/252), the 1930s meaning of this root may have survived, but it is hard to be certain since the name was only properly translated in the 1930s.
The 1930s root also had an unaugmented variant ᴹ√LAK with derivatives ᴹQ. (a)larka, N. lhagr “swift, rapid” (Ety/LAK²). Whether this unaugmented variant remained valid is unclear, but there is nothing in Tolkien’s later writing contradicting it either.
The root ᴹ√RUS “flash, glitter of metal” appeared in The Etymologies of the 1930s with derivatives ᴹQ. russe “corruscation, †sword-blade” and N. †rhoss “polished metal” where it served as the basis for the second element of the name Maidhros “Pale-glitter” or (Anglicanized) Maidros (Ety/RUS; MAD). In The Shibboleth of Fëanor from 1968, the second element of the names S. Maedros and S. Amros were explained as S. ross < ✶russā referring to their red-brown hair (PM/353, 366; VT41/10). A related etymological note stated:
> Common Eldarin (u)rus [was] used of a varying brownish red from what we should call brick-red to auburn. Hence Quenya, Telerin urus (stem urust-), Sindarin rust “copper”, rustui adj.; Quenya {ruska “red-brown”} rusko “a fox” (rusku-, pl. rusqui; ruskuite “foxy”). (calarus(t)- polished copper, lairus(t) verdigris). russe a head or pelt of red hair, russa red-haired. S. rusc fox, ross ([primitive] russā) red-haired, copper coloured, especially used of animals, as fox, red deer, and [?similar kind] (VT41/10).
This use of √(U)RUS as brownish-red is not entirely without precedent: in The Etymologies of the 1930s Tolkien gave the “root” (more likely just a primitive form) ᴹ√RUSKĀ with derivative N. rhosc “brown” (Ety/RUSKĀ), the basis for the name N. Rhosgobel “Brownhay” from Lord of the Ring drafts from the 1940s (TI/164), later translated as “russet village or ‘town’ (enclosure)” (RC/241). However in notes also associated with The Shibboleth of Fëanor, Tolkien gave the root √URUN = “copper” apparently as an extension of √RUN “red, glowing”, part of an explanation of the sobriquet of Nerdanel’s father: Q. Urundil “Copper-lover” (PM/366).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best to assume 1930s ᴹ√RUS “flash, glitter of metal” was discarded in favor of 1968 √RUS “brownish red”. I’d also use √RUS as the basis for copper words rather than √URUN.