Although ancients believed the sun brought knowledge as well as light, the brightness of the sun bringing brightness of the mind is a reasonable explaination for yellow signifying the mind and the intellect.

There are many shades and tones of yellow and gold, providing numerous associations, notably with objects and effects found in nature, including light, the sun, and flowers. 

The heraldic use of yellow to indicate honor and loyalty is contradicted by later connections with cowardice. 

The brightest color in the spectrum, yellow is seen before other colors when placed against black. This combination is often used as a warning sign - in nature, by insects such as bees, and to signal hazards in industrial situations.