Thank you for contacting us.
There is an issue with the disabled state of RadDropDownList background indeed. By design, when the control is disabled its whole BackColor should be light blue. I have logged it to our Public Issue Tracker under ID 12385. To vote for it and subscribe for its status updates follow this link: http://www.telerik.com/support/pits.aspx#/public/winforms/12385
The issue is already addressed and it will be part of our next official release Q3 2012. Your Telerik points have been updated.
As a workaround I suggest using the attached modified theme files. Here are several articles that will assist you in the process of loading and applying the themes.
Please note that when the control is disabled and is in DropDown mode, there is no way to change the color of the font through theme. This is a known limitation that currently can be avoided only through manually setting the TextBoxItem's ForeColor at runtime.
I hope you find my answer helpful. Let me know if you encounter any further issues. I will do my best to assist you.
All the best,
the Telerik team