An adjective in The Etymologies of the 1930s glossed “of gold” based on N. malt “gold (as metal)” under the root ᴹ√SMAL “yellow” (Ety/SMAL). The form was revised to (h)malthen indicating an archaic voiceless hm that was the result of ancient sm (EtyAC/SMAL), but in Tolkien’s later writings the root became √MALAT (PM/366) so this would no longer be the case.

Neo-Sindarin: In Sindarin of the 1950s and 60s, medial lth became voiceless ll (VT42/27), as seen in the noun mallorn “gold tree” and (probably) the later adjective mallen “golden”. However Tolkien said:

... among those to whom Sindarin became a language of lore, as the men of Gondor who were or claimed to be of Numenórean race, the spirant [th] was reintroduced from the spelling. In true Sindarin of the Elves or Elf-friends of the early ages the final form [lt] was often introduced medially (VT42/27).

As such, I think malthen “of gold [metal]” may still be viable by analogy with its noun form malt “gold [metal]”, and the adjective mallen may be limited just to golden colors.

Conceptual Development: The Gnomish Lexicon of the 1910s had similarly formed G. maltha “golden yellow, rich, mellow” (GL/56) likely based on the early root ᴱ√MALA² “yellow” (QL/63).