The numeric scale does not support the ranges visualization as linear or radial scale, but it supports the range list which contains RangeBase class instances. So, you can use them to specify the range (min and max) and the IndicatorBackground property.
At present, the numeric indicator does not support using range colors directly, but as workaround you can use an IndicatorBase class instance and then bind its background to the Foreground property of the Numeric Indicator.
We have created the PITS issue to implement this feature in future releases. You can track it using the following link:
I have attached a sample solution which changes font color of Numeric Indicator base on the ranges.
the Telerik team
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to help you get started with RadControls for Silverlight