As an adjective, "golden" usually refers to the colour. -> "They sat and watched the golden sunset." "He wore a golden ring" = "His ring was the same colour of gold but I don't know if it were really gold... it could have been brass."
"Gold" is a noun and should be used only to refer to the metal. -> "He wore a gold ring." = "He wore a ring [[that was] made] of gold."
Compare with silver/silvern; brass/brazen; iron/ferrous, etc.