When customizing the Global Navigation Bar there are two options - change the Logo or the Bottom Border Color. Changing the Logo will also change the background color of the Global Navigation Bar to light gray. The color code of the light gray is as follows - #EEEEEE or (238 238 238) in RGB. This color is set through the code and is not customizable.
The Bottom Border Color can be customized and will not affect the background color of a Global Navigation Bar.
Please refer to the following screenshot for the showcase of this functionality.
The following help article contains more information on Branding the Workfront Site.