This article explains which files to edit in order to add a custom set of colors to a Roundcube Plus skin. Please note that in order to make these modifications you'll need to have some programming knowledge.

Let's use the Outlook skin as an example.

The color settings are stored in the file skins/outlook/assets/_colors.scss. Edit this file and modify the values in the $map section. The column on the left represent color keys, and the values on the right represent colors values. Each row represents a single color scheme.

Choose one of the rows and modify the colors to correspond with your company's branding.

Save _colors.scss and compile the files desktop.scss and mobile.scss. The instructions on how to compile them can be found here:

http://sass-lang.com/

If you change the color key (the color in the left-most column of _colors.scss,) you will also need to modify the color array in settings.php to reflect your changes.