2. Observer (I see, I see what you don’t see)
Also remarkable; people are often the weakest link in evaluating a color difference. Inter-observer variability is really fascinating subject!
Just take the three types of cones in the retina of our eyes. Who would ever considered two people with “normal” trichromatic vision could differ in totally different ratio’s between the red and green sensitive cones (79:20 versus 51: 44)?
Apart from physical differences and (dis-)abilities due to age, eye sight or color blindness, people can also be influenced mentally. Just mind-set, mood ,stress, tiredness or using drugs or medicines can affect their decision on whether two objects appear to be similar or not.
People can also be trained to evaluate colors in a quality control setting and develop a higher sensitivity towards visual differences. Professionals in visual quality control are often taught to describe the difference they perceive in hue, lightness and saturation and somehow this regular stimulance makes them super critical which may both work positive as negative in applying their job.