We all may be looking at different colors.
Since you were born, someone told you that the color you were looking at was, for instance, blue. So the color in your head is blue. But what if the color you actually see is red. But since someone told you it is blue since the day you were born, you call red, blue. The same could be true for every color you see. The color you call green could be pink, or orange could be purple. The same could be true for all of us. And there is no way we could ever prove otherwise.