Hello Dennis,
The RadSlider control is a complex control, not a simple HTML element and the HTML it renders is consisted of a lot of HTML elements. That is why, in my opinion, it is not clear to which element of the RadSlider HTML the ForeColor property should be applied. That is why, it is ignored.
In case you what to customized the look of the RadSlider, you can do one of the following:
Create a custom skin. You can set the Skin property of the RadSlider in the code behind. For additional information on custom skins, please refer to our online documentation.
Use the CssClass property of the RadSlider to apply changes to the look of the RadSlider in case you do not intend to create whole custom skin, but only what to change the image for the selected region element. For example:
I have attached a simple test page that demonstrates the second approach - how you can switch the color of the selected region from blue to red dynamically by using the CssClass property of the RadSlider.