Answer:
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 .