This adds a new "Project Colors" section in data defined buttons which are linked to a color value. The color menu contains all colors defined as part of the current project's Project Color Scheme (which is defined through project properties).
When a project color is selected from the button, the property becomes linked to that color. It will automatically follow any future changes to the color when made through project properties.
This allows users to define common colors for a project once, and then "bind" symbol, label, layout, etc colors to these preset colors. The link is live, so you change it once, and the change is reflected EVERYWHERE. Sure beats updating a color 100 times when it's use has been scattered throughout a project's symbols, labels, etc...