What is a yellow light?
A yellow light is a statement of limited approval or permission to proceed on a task, or the light seen between the green and red light on a set of tricolour traffic lights.
Even though youv'e given it the name "Yellow", the object is only yellow when there's some yellow light shining on it. You see, it absorbs all other colors of light, and only yellow light bounces off of it. That's why the light from that object that enters your eye is yellow light, and that's why you describe the object as looking "yellow". If there's no yellow light shining on the object, it looks black.