Tolerancing color in presswork by eye

Once the press operator has achieved their solid ink density targets during make ready, they typically will do a visual examination to compare presswork color to proof to evaluate the closeness of the match. They will also do visual comparisons during the run to check the consistency of the match through the press run. Print buyers typically also do the same thing - relying on their eye for color to verify the match and consistency.
For most people in the graphic arts, the eye is the final arbitrator on the quality and consistency of presswork - however, this method has some limitations caused by the fact that the eye is part of a very tricky instrument: the human brain. For example, look carefully at this graphic containing light green and light blue swirls:
Light green and light blue?

Actually there are no light blue swirls, What you see as light blue is actually the same color as the green ones. They are both R 0, G 255, B 151. Cutting out a section of the "light blue" swirls and lining them up with the light green ones proves they are indeed the same color.This illusion is so strong that you might have to down load the image into PhotoShop and confirm it for yourself.

There are several characteristics of our eye/brains that can play tricks on our perception. Being aware of them will help you more clearly understand how your color perception can be mislead and hopefully provide a clearer view as to what you are actually seeing.

1) The eye/brain auto-white balances. The eye/brain selects an area that it "knows" is white - forces it to appear white and balances other colors accordingly. This is the trick that allows us to see a white paper as white with surrounding colors being natural under a variety of different colored lighting situations. This often causes problems with monitor proofs where an image should be "white" i.e. should result in no halftone dots in the presswork, but in fact has grey or even a color cast in it that results in dots being printed. The eye/brain sees the area as white when in fact it is not.

2) The eye/brain has no color memory. Not only does the eye/brain auto-white balance, it also rebalances color whenever you look from one object to another. This makes comparing two colors that are separated, by even a small distance, impossible. For example, in the below image, the press operator cannot effectively compare color between the image on his soft-proof with the color on his press sheet.
The only way to compare two colors is by cutting through one sample and overlaying it on a reference (e.g. cut press sheet over proof) like this:If the color aligns across the cut then you have a match.

3) The eye/brain cannot judge variation consistently. In order to tolerance acceptable color variation, for example custom/brand colors, you need to have a reference high/low density guide that provides an example of the two extremes that the color must fall within. Providing a swatch guide with holes through it as in this example:allows users to place the swatch over the press sheet to more easily confirm the color match as well as whether it falls between the two acceptable extremes.

4) The eye/brain's perception of color is influenced by the size of the area of color. This is one of the reasons that the paint color selected from a small paint chip seldom appears the same as the color once it's painted on the wall. The same issue happens when selecting a spot or brand color from a swatchbook. Always try to get a reference chip, draw down, or previously printed sample, that is as large as possible.

5) The eye/brain's perception of color difference is not uniform for all colors. It is difficult for the eye/brain to see differences in highly saturated colors. However, a small degree of variation is easily seen when colors are near neutral. Variations in the green part of the spectrum are more easily noticed than the same degree of variation in the red part of the spectrum.