This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme of RadMaskedEditBox.
Open VisualStyleBuilder: Start menu (Start >> Programs >> Telerik >> UI for WinForms [version] >> Tools).
Export the built-in themes in a specific folder by selecting File >> Export Built-in Themes.
Load a desired theme from the just exported files by selecting File >> Open Package
Select RadMaskedEditBoxElement in Controls Structure on the left side and then select its ForeColor property.
Modify the applied repository item.
You can see the result directly in the Visual Style Builder.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadMasedEditBox by using the demonstrated approach in the following link: Using custom themes