Yellow is happy.
It's the sun. It's bright.
It's also the color of lemons.
Years ago, a businessman came to me asking me to do a flyer for him. He didn't care too much about how it looked, so long as it was printed on yellow paper. That's because yellow stands out in the sea of white and not-so-white papers that everything else is printed on. Like all warm colors, yellow pops out at you. Danger signs and for sale signs are yellow for a reason.
Yellow is also the color of gold. And gold spells luxury.