A brand manual should contain all definitions for the color systems the company or their service providers will eventually be working with.

CMYK is used for print products, RGB & hex# for digital screen displays. RGB and Hex both define the same thing in a different way. Defining both will ensure that the color is always as expected no matter if the digital product is created with RGB or Hex colors. A print color can but does not have to be a Pantone color. The print products the company will produce on their own can be printed in the Pantone colors (pre-mixed colors that are used for the print). If the company cannot influence the print process (for example when placing ads in a magazine) then their Pantone color will be printed in another system, which could end up to be CMYK in which the final color is a result of the subtractive mixture of colors during the printing process rather than using a pre-mixed color. For this reason it is necessary to define a CMYK color. 

I do not think there is a rule or best practice but that it depends on the brand's requirements. 

Thank you for your response in which I fully agree. I teach in these subjects.

I am working on a Paper where I’m arguing against this inappropriate way of specifying Brand Colors in Brand Manuals. Firstly there are no information on what kind of RGB the Brand Manual is displaying (sRGB, AdobeRGB, eciRGB, AppleRGB etc.).

And the same goes for all the other color specifications.

Thus, I’m wondering from where the Graphic Designers and Brand Owners get their inspiration of specifying colors. Are there a golden book of best pratcise or some Papers on all this?

Thanks again

I think the inspiration either comes from a close examination of the eventual cases or from "because it's always been done in this format". The lack of RGB info is a good point. But maybe this is the case because additive color mixtures are wild and free. I work solely in digital media and have to live with the fact that the perceived color will be different on every monitor in every individually lit room at various hours of the day...no matter how many color systems are defined. 

I think that the best practice would be using the files in CxF (Color Exchange Format). This format has become an ISO standard (ISO 17972-4: 2015) and is dedicated to the exchange of information on color (color communication) and seems to be ideal for determining and describing Brand Colors.

Thank you for your previous answers and interest. I have now uploaded my paper to my profile

There is no standard for this simply because there is not colour matching system that covers all possible media and printing processes. The right way to define a brand colour is for the designer to decide at the creative stage which media should be used for his brand colour(s) and then define a complete SET for each colour, using existing and commercially available colour systems and standards - see www.spot-nordic.com/defacto.

https://www.spot-nordic.com/defacto

Michael Abildgaard Michael, All the color guides supply formulas for reproducing color on a specific substrate. They also tell us what light source to use to determine a match. There is no literature of this because these “guides” have been successful in helping us match color for centuries. — With regard to your question about what kind of RGB. the literature (color science) exists, is easily found on line and printed literature is extensive, overly detailed and addresses the calibration of the source and tools to match color.

People who use color for tens of centuries have asked, How can I make/mix that color?” — Its why the cave paintings have similar color and last a similar amount of time. —All who at one time or another…what to make/re-make/copy the amazing color they saw. ——— Each practice that uses The closest source of literature, that is; “inspiration of specifying colors**” color has its own technology for sourcing and making color for the natural and synthetic dyes pigments and minerals.

For example; in the fashion industry its common to apply a color used on silk on cotton. This surface mis-match is made possible by using the extensive **color cards and **sample books made by the textile dye and pigment color manufactures. Correspondingly each industry publishes it papers in their related journals and presents new research and techniques at trade conferences.

As our tools and technology develops wider gamuts, brightness and tone each of the “brand manuals” get (constantly) updated. Plus we add new ones; for example metallic, florescent, 3-D and laser light.

Lastly, its “best practice” because it its been proven by each new generation of colorists applying and making color using each new generation of tools/ technology, to work.