Vignette
Vignettes or subtle radial gradients are used when there is a need to draw the audience's eye inward to contain its focus on something in particular.
Amount
Subtle. Done as a gradient, color value transitions should feel natural and not artificial.
Best Used For
Flat colored backgrounds such as red, white or gray. Screen applications (websites, presentations) benefit most from this graphic styling as it reduces the harshness of a bright-white background in particular.
2008-2011
In the initial iteration of the brand platform, there was no depth to any background. Everything was pristine and flat.
Present
While flat-color backgrounds are still acceptable in the current brand platform (when appropriate), vignettes or subtle radial gradients are used when there is a need to draw the audience's eye inward to contain its focus on something in particular.
Below are some examples of current usage:

