The human eye can distinguish millions of colors. For finishers seeking to consistently match a color standard, the human capacity to discriminate even minute color fluctuations makes getting the color exactly right a quality control priority.
Color differences can result for many reasons, ranging from coating formulation inconsistencies to variations in the application process. As a result, consistent color matching requires diligence and teamwork on the part of the coating supplier and the finisher, with both taking steps to ensure consistency from batch to batch and substrate to substrate.
Color space is the term used to refer to all of the colors—or light wavelengths —perceptible to the human eye. Color is measured using a spectrophotometer, an instrument that measures light at many points along the spectrum. The results are used to calculate a mathematical curve that indicates the combination of light wave components making up the complete color—in other words, the color's blueprint.
The most highly sophisticated spectrophotometers view light at multiple angles as well as at multiple points along the spectrum. These are especially useful for comparing colors containing metallic and mica flakes.
Measuring and Describing Color
Color differences are measured in units known as "delta E's," indicated symbolically as DE. One DE is the smallest color difference perceptible to the "average" human eye when colors are viewed adjacent to each other. Among the difficulties in measuring and describing color, however, is the fact that everyone's eyes work differently. At one DE, some people may see no color difference at all. Or, depending on their eyes, two people looking at the same color sample may see it quite differently. One may see the color as having a bluish cast, while to the other, it may appear yellowish. In general, as the human eye ages, colors appear increasingly yellow.
Describing subtle differences in color can be difficult, since people may attribute different meanings to words. Hue is generally accepted to describe color families, such as red, blue, yellow, green, orange or purple. Chroma, or saturation, is generally used to describe a color's depth or intensity. The word "shade"—as in "a shade lighter" or "a shade darker"—is often the laymen's way of describing a difference of one DE in saturation. Since spectrophotometer measurements are more precise than language, record keeping and communication about color should involve use of this data where practical.
Pigments and colorants are the primary ingredients that give coatings their color. Pigments are chemical compounds that reflect only certain wavelengths of visible light. Even more important than reflecting light is the ability of pigments to absorb certain wavelengths of light.
Color matching is easier when the exact same pigment is being used to match the original color standard, but with so many varieties of pigments on the market, it may not always be possible to match pigments exactly, especially when coatings technologies evolve or different coatings manufacturers are used as suppliers. For example, some paint manufacturers no longer use pigments containing chromates, so original standards containing pigments with these components are more difficult, but not necessarily impossible, to color match.
Paint manufacturers can do a great deal to ensure consistency of color from batch to batch. The set of additives to be used in a coating, such as pigments, colorants and shading bases and their specifications, are part of a coating's color formula, which is kept on record. During the manufacturing process, great pains should be taken to ensure, through meticulous methodology and precise instrumentation, that all components imparting color to a coating meet their pre-determined color specifications. After manufacture and before the color is put in its shipping container, color readings should be taken and compared to pre-set color specifications, then recorded and filed for that particular color.
In addition to slight variations in color components from manufacturer to manufacturer, there are many application factors that can cause color differences. For example, oven temperature can cause color to shift. Even on the same part painted with the same coating at the same time, color variations may result, often from differences in either film thickness or substrate thickness. These differences occur when the surface temperature of the heated part is not uniform due to thickness differences, even though the oven itself may heat up evenly.
Over time, color shifts occur even at room temperature and with exposure to UV light. Also, gloss variations can cause changes in the appearance of a coating color. Gloss can vary for many reasons, including a change in the surface profile of a part. To ensure the integrity of original and master color standards, they should be placed in plastic, zip-lock bags and placed in frost-free freezer storage at controlled temperatures of 0–10°F. To prevent condensation upon removal, the samples should be allowed to warm up to room temperature before the bags are opened.
Working color standards should be kept on-site at both coatings manufacturer and finishing shop locations to be used for agreed-upon quality control procedures. These procedures should involve either making visual matches to the color standard or measuring and comparing color matches with a spectrophotometer. Batches that are off color should be rejected.
Investigating Color Differences
The first step in identifying the cause of color differences is to determine whether the problem is with the application process, the measurement process or the paint formulation. For coatings manufacturers and finishing shops to work together on color matches, their spectrophotometers must be calibrated to each other. Often, fixing a color match is as simple as synchronizing spectrophotometers. Spectrophotometers should be calibrated regularly, either daily or at the onset of each shift.
Aside from the causes of color shift described earlier, other common reasons for color differences are equipment problems including trouble with pumps, or leaky valves or seals. Sometimes a catalytic reaction can affect gloss or pigments. At times, multiple causes are responsible for color changes. Any investigation into color differences needs to be systematic, deliberate and thorough so that one by one, potential causes can be eliminated.
Using Multiple Colors
Given the appeal of colorful products in both the consumer and industrial marketplace, it's not surprising that many parts require the application of multiple colors, sometimes in intricate patterns. Trucks and toys, for example, may have as many as seven or eight colors applied to a single part. Masking is critically important to achieving clean edge lines when multiple colors are used on a part.
When frequent color changes are required on a finishing line, paint can be wasted. To minimize paint waste, batch all parts requiring the same color so they can be painted at one time, reducing the number of color changes. Also, consider running small diameter lines from the color manifold to the application equipment, and shorten the distance and the line length between the color manifold and the application equipment.
Getting Help With Color
Coatings manufacturers can provide a great deal of help with color selection as well as color matching. Because color can have such a dramatic impact on a product's appearance, it is important to make informed decisions regarding product color. Product designers may benefit from consultation with color professionals.
Color experts are skilled in the language of color and bring insight into its uses. They have an understanding of the interaction of color and lighting, color and the emotions and the color trends affecting various industries, cultures and regions. These specialists can help OEMs evaluate an existing color palette and make recommendations based on forecasts specific to industries ranging from metal office products to electronic enclosures, building products and home furnishings. They can help to name and organize colors on a color card to ease its use, and they can work with OEMs and job shops to establish a comprehensive color standards program that ensures perfect color matches batch after batch.