Color management in ArcGIS Pro ensures that the colors you use in your maps, scenes, and layouts appear consistently across devices: for instance, on a different monitor, when exported to a PDF, or printed. Without color management, the devices that display your work do their best to approximate your chosen colors, but getting faithful color matching is unlikely. Color management can't guarantee exact color matching but ensures that the fidelity of color is maintained where possible throughout your workflow.
A color management system achieves this by adding information to the color definitions and images in maps and layouts. This extra information is stored in a file called a color profile. Color profiles provide a reference for the color values in your project specific to the devices you are using for display and output. The color management system uses these profiles along with a set of conversion policies to ensure color uniformity when colors are moved into different project items and viewed on different devices.
Enable color management whenever color consistency is necessary or when your output workflow requires an embedded color profile for use elsewhere, such as when sending the file to a printing bureau.
Why is color management necessary?
Discrepancies can occur when you define colors in one place and understandably expect to see the same color in another place. Ideally, a color in a map should look identical whether on your computer monitor or printed on a piece of paper. But color model values are interpreted differently by different devices, such that the same color values may appear differently depending on where they are viewed. Often the differences are subtle, and in many cases, they may not matter. But in situations in which it does matter, you must use color management to control how devices interpret and output color, thereby ensuring consistency.
One reason that it is challenging to get colors to match across devices is that some devices, such as a computer monitor, use light to create color. These devices rely on additive color models, in which different channels of light (usually red, green, and blue, or RGB) are mixed in different proportions to make visible colors. RGB is measured in increments from 0 to 255. Full proportions of all three light channels (RGB 255,255,255) add up to a fully white screen. No light from any channel (RGB 0,0,0) results in a fully black screen. Different proportions result in different hues. When all three channel values are identical, the result is a tint of gray.
In comparison, a printer uses ink to put color on a page. Three channels of ink, usually cyan, magenta, and yellow (CMY), are mixed in different proportions to make visible colors. Printers are subtractive systems. No ink from any channel (CMY 0,0,0) results in a white page (assuming white paper to begin with, of course). Full proportions of all three channels result in black—or more accurately, near black. It is difficult to get a deep black color by blending colored printing ink. For this reason, printers usually have a fourth ink channel of black (usually abbreviated as K) to ensure that blacks are sharp.
Even though RGB and CMYK are conceptually opposites, they do not occupy exactly the same volume of color space. The entire volume, or domain, of colors that can be defined by a color model is called the gamut. The gamuts of RGB and CMYK have most of their volumes in common, but there are some differences. Color management can help determine what happens when a color is defined in one model, but then converted to and output to a model where that color is out of gamut. In ArcGIS Pro, each color-managed project item has two defined color profiles, one in RGB and one in CMYK, and a color model set for one of the two.
The numeric definitions of a color in these models seem very accurate and exact, so it's easy to assume that one set of color values will render identically everywhere. But they are really just values that only have meaning in the context of the color space in which they are defined. In this way, they are synonymous to map coordinates. Map coordinates are only meaningful in the context of a particular coordinate system and geographic datum. Without that reference, they are just arbitrary values. Color profiles give that context by assigning meaning to color values.
A color space is a specific instance of a color model that has a defined gamut. So, there can be different instances of RGB models where they all define and classify colors with RGB axes, but each may have slightly different gamuts of the range of colors they define. Each device has a color space. No device can capture or reproduce every conceivable visible color. The range of colors that a device can reproduce is its gamut. When a color moves from one device to another, the visual appearance of it may differ because the new device is interpreting the color values in reference to its color space. Color profiles are used to translate between devices to ensure color consistency.
A chromaticity diagram is based on measurements of how the human eye perceives light. A flattened view of this 3D volume is shown below. The curved line plots individual wavelengths of light, or spectral colors. All colors perceptible to humans with normal color vision plot somewhere inside this curved shape, including purple and magenta, which are not spectral colors but mixtures of red and blue light.
Chromaticity diagrams are useful for comparing the gamuts of color spaces. In the following diagram, the colored triangle is the sRGB gamut, which represents the colors that can be displayed on most computer monitors. Due to the flattening of the diagram, only the full-intensity colors are shown here. All others plot conceptually behind the triangle, out of view in this diagram. The SWOP CMYK gamut is outlined in white. The areas where these gamuts do not overlap represent colors that are displayable in one color space, but not the other. This illustrates how CMYK cannot display many of the violets and greens that RGB can.
A color profile is a description of the color space and gamut of a device as a context for color values, in a format that a visualization system can understand. The visualization system uses color profiles to convert color values to achieve visual color consistency from device to device. Values get converted to the Lab or XYZ color model as an intermediate step. Lab defines colors using values based on visual perception, not values that refer to an amount of light or ink. For this reason, the Lab model is device-independent and can be a neutral intermediate step for color conversion from one device to another.
The color management system ensures color consistency by translating colors using information in the source and destination color profiles along with the color conversion rules defined in the application as colors move from project item to project item or from device to device.