The Telerik controls use different CSS classes when they are in disabled mode. So there's no universal setting that can target all of them. You will have to override the CSS class for each control for which you wish to change the appearance in disabled mode.
For example all input and picker controls (RadNumericTextBox, RadDatePicker etc.) internally use RadInput so you can use the following CSS rule to change the color of the disabled text in Safari and iPad:
And for RadComboBox you can use the following:
I hope this helps.
the Telerik team
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