I am afraid that RadGrid from the Telerik UI for ASP.NET AJAX suite does not provide APIs to enable/disable colors the same way you would do in a WinForms project. If you want to change any of the colors you will need to use CSS as this is a Web-based application.
In case you would like to have a UI that allows the users to change cell colors, that will need to be implemented additionally.
For Web-based applications, I would suggest inspecting the HTML elements and the styles applied to them. You can follow the suggestions in the first two points of the Improve Your Debugging Skills with Chrome DevTools blog post explaining how to inspect the generated HTML and check the applied styles for various elements.
Once you know the styles you need to override, you can use the same style selector and add "html body " in front of it to make it more specific, "stronger". More on CSS specificity you can find here:
To learn more about CSS styling and using the Browser's developer tools, you may find the following videos useful:
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