Yellow is associated with jealousy, envy, betrayal and cowardice as well as optimism, sunshine and pleasure; the yellow pigment on the horse in the Lascaux cave is estimated to be over 17,000 years old; Vincent van Gogh famously sung the praises of this sunny hue in a letter he wrote from the south of France to his sister in 1888. (via)
Aequam memento rebus in arduis servare mentem – Horace