This was the root for Elvish words meaning “yellow” for much of Tolkien’s life, though with some minor variations. It appeared as ᴱ√MALA “yellow” (usually mali-) in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. malina “yellow” and ᴱQ. malikon “amber” (QL/58). It also appeared in a list of M-roots at the end of that section (QL/63). It had derivatives in the contemporaneous Gnomish Lexicon such as G. malon “yellow” and G. malthos “butter cup” (GL/56).
In The Etymologies of the 1930s it seems Tolkien first gave this root as ᴹ√MAL (EtyAC/MAL) but rejected this and replaced it with ᴹ√SMAL “yellow” (Ety/SMAL). It had derivatives like ᴹQ. malina/N. malen “yellow”, ᴹQ. malta/N. malt “gold (as metal)” and ᴹQ. malo/N. hmâl “pollen, yellow powder” (< ᴹ✶smalu), with some revisions in Noldorin forms as Tolkien vacillated on whether or not primitive sm- resulted in voiceless nasal hm- or a voiced nasal m-.
This √SMAL vs. √MAL variation seems to have continued into Tolkien’s later writings, as seen in Common Eldarin: Noun Structure from the early 1950s where ✶malu >> ✶smalu “dust, grit” (PE21/80), probably a later iteration of ᴹ✶smalu “pollen, yellow powder” from The Etymologies. But it seems Tolkien settled on √MAL as evidenced by the extended root √MALAT “gold” from The Shibboleth of Fëanor from 1968 (PM/366).
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.