You can use Telerik RadColorPicker. Telerik RadColorPicker for ASP.NET AJAX is a lightweight UI component that allows users to select colors from the RGB or HEX color spaces using a configurable palette view.
According to your description and needs, I checked it in my side of you provided site. I found a samples about how to achieve this in C#, please check:
Asp.net: change the text color in Image dynamically: