My father was a photographer and over the decades he worked with many other photographers in his darkroom (this was in the era of chemical photography). After several hundred occasions when he and another photographer were looking at a print under the color balancer, he realized they were literally seeing different colors. My father, who was scientifically inclined, eventually worked out the average variation, which I want to say was two clicks on the projector, but I’d need to check to find out what that actually meant in terms of frequencies. It’s a small variation, but it was certainly real. Remember, these were all professionals with years of skill at judging color.