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.
The root √KUL was connected to gold and gold-coloured things throughout Tolkien’s life but gradually shifted in meaning. The earliest form of this root was ᴱ√KULU “gold” from the Qenya Lexicon of the 1910s (QL/49), but at this early stage it was connected to actual metallic gold as much as the colour, as opposed to ᴱQ. laure which was “magic” name of gold (LT1/100). In The Etymologies of the 1930s, ᴹ√KUL was first glossed “gold (metal)”, but was eventually revised to “golden-red” (replacing a rejected variant ᴹ√GUL), and metallic gold became ᴹQ. malta < ᴹ√SMAL “yellow” (Ety/SMAL) while ᴹQ. laure shifted in sense to “golden light” (Ety/GLAW(-R)).
Tolkien’s continued use of names like Q. Culúrien (S/38) and Q. culumalda “tree with hanging yellow blossoms (prob[ably] a laburnum)” (RC/626) indicate the continued validity of this root, though I suspect in later writings it primarily referred to a golden-red or orangish colour.