Generally, you need to drag and drop and then nest the following elements from the "Assets" toolbox (in that particular order) to achieve the desired effect:
As for customization of the gauge color -- unfortunately currently there is no API exposed for easy modification of the gauge background color and you will need to edit the RadialGauge template
as actually the RadialGauge template defines all the complex visual effects that you are seeing in the default theme of RadGauge.
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