John is spending the day at the beach. if he stares at the blue sky for a long time and then look at his white beach towel, what color will he see, according to the opponent-process theory of color vision?

Yellow

Explanation:

The opponent-process theory -

The theory was given by Ewald Hering ,

The theory states that , the cone photoreceptors are connected to each other in the following way -

black/white ,  red/green and blue/yellow .

Hence , by absorbing a particular color the other is reflected i.e. , observing one color for a long period of time inhibits the color and we see the other color .

Hence , by observing blue sky for longer time , we see yellow color .