Standard RGB Color Spaces

This section looks at the relative merits, as well as some of the shortcomings, of popular standard RGB color space profiles.

Comparison of spaces

The are several RGB spaces that are formal color space standards. Each can be used in Capture One for color correction and when using color readouts for comparing with reference values supplied with reflectance charts (i.e., color targets). These include sRGB, specified by HP and Microsoft (IEC 61966-2-1:1999), Adobe RGB (1998), ProPhoto RGB (ROMM), specified by Kodak (ISO 22028-2:2013) and ECI RGB v2, specified by the European Color Initiative.

One of these color spaces should be used when editing and using the Lab color readouts in Capture One. The color space profile can be selected using either the proof profile option or from the selected process recipe (verification required). Note that Capture One does not edit in this space, instead it determines the color it would use if it processed the image to a file. When the process recipe option is being used to determine the space, it is this same space that’s used when being processed for output. Therefore, the proof profile option should be used with some caution if an output file is required for further validation, in case its processed with a different color space profile.

RGB color spaces vary in gamut, gamma tone curve and white point or media white, and the profiles used to describe those characteristics are open to interpretation by different color engines when converting to the Lab color space. While that’s not an issue if you only ever measure RGB readouts in Capture One, besides taking care to match the RGB space of the chart’s reference values and the output RGB space, this can cause a mismatch if comparing the Lab readouts in Capture One with the color values in the output file using a third-party application.

sRGB

This color space is an important space, for several reasons. Not only is it the standard for the web, images that do not have an embedded color profile are assumed to be in sRGB, many monitors approximate the space or adopt modes that do. Besides a relatively small gamut, limitations with this profile include an increasing number of profile versions that claim to adhere to the standards but deviate in some way. Some of these, for example, include a model for flare, which should be avoided. The profile shipped with Capture One does not include a model for flare. Even the ICC profile differs from the original standard in not having a neutral white, and doesn’t adhere strictly to the media-white of D65. This has led to some third-party color engines, notably ACE to detect errant profiles and "hot-fix" them, neutralizing the white-point. The many differences in these profiles may affect comparisons in Capture One with analysis software, not only in those interpret images directly (without the ICC profile - such as ISA GoldenThread) but also those that silently fix them.

Adobe RGB (1998)

With a relatively large gamut that encompasses many printer and some high-end monitor spaces, along with tightly controlled profiles, this is considered a useful working color space. Although white is exactly neutral, there are still a few potential pitfalls for color engines to interpret the results correctly. The media white point is specified as D65, which may require chromatic adaptation, and the ICC profile adopts a gamma tone curve that’s subject to slope limitation. This can cause dark and saturated colors to be misinterpreted between applications.

ProPhoto RGB

Initially, this space looks like a useful working space to use in a reproduction workflow. It has a gamut that’s large enough to encompass a practical range of colors used for both digitization and printing, a white-point specified as D50, thereby avoiding chromatic adaptation, and a gamma tone curve with a linear segment for dark tones that color engines can interpret. In practice, though, even ProPhoto RGB has some limitations. The initial ICC profile from Kodak is subject to slope-limiting, and subsequent versions neither adopt D50 as the illuminant, nor detail white as neutral. A more recent version released by the ICC includes a flare model, which should be avoided. Finally, the large gamut means that there are large gaps in 8-bit form between the 256 steps that can lead to posterization, therefore files should be maintained at 16-bits/channel throughout the workflow.

ECI RGB v2

This color space is the recommended space in the Metamorfoze Preservation Imaging Guidelines, and the only one allowed at the highest level of those imaging standards. This profile is the most likely to produce the same result across different color engines. Although white is not neutral (it has a slight tint at 255, 255, 255), it’s the only profile that avoids all the other compatibility issues. The space has a usefully wide gamut that extends beyond AdobeRGB in certain hues, however, it cannot quite represent saturated blue and magenta hues that sRGB can. Nevertheless, it is particularly suited to modern printers and, with correct exposure, uses bit-depth efficiently, making it one of the most suitable color spaces for 8-bit/channel images.