The meaning of gold
Some Noh masks have gold-coloerd parts. In the field of Noh mask, the color of gold means "They are not human". Let's check it out more.
The color of gold is unnatural color. So it is used for God, ghost, monster and beast. So if you find a gold parts on the Nohmask, you can tell it's not a human nature.
It's interesting to know that the mask "Deigan" looks like a woman. But if you look at the mask carefully, you can find she has golden tooth and eyes. The reason is that she has a strong feeling of jealousy, envy and anger. So Deigan implies she is turning into a monster. Deigan will eventually turn into "Hannya". Hannya also has gold eyes and tooth.