"Es una llave de oro."
Translation:It is a gold key.
Your description is a good way for an English speaker to remember how to read the Spanish, but I think it is not completely true that "golden" only means the color in English, especially for non-literal phrases, e.g. "goose that lays the golden egg" is a metaphor for something that provides a highly valuable service or product, such as an egg made of gold (whereas a gold-colored egg wouldn't be so valuable); similarly "golden parachute" refers to an exit scenario that makes one rich; the "golden rule" is the most valuable rule, as though it were made of gold.
Not quite, Iago is more correct in this case since the eggs would be golden even were they painted that color, and the golden parachute is a metaphor I do not think corresponds to a real world example and would still be true even were the parachute woven of some material dyed that color. You are right to the extent that things OF gold are golden, but not right that golden denotes 'made of gold', otherwise Charlie wouldn't have needed to go to the factory, he had a golden ticket.
All that glitters is not gold, but all that is golden is of that color.