Thank you for writing.
Please note that the auto-hide tabs are dynamically created. So you can change the color when a window is hidden. For example:
radDock1.DockStateChanged += RadDock1_DockStateChanged;
sender, Telerik.WinControls.UI.Docking.DockWindowEventArgs e)
(e.DockWindow.DockState == Telerik.WinControls.UI.Docking.DockState.AutoHide)
e.DockWindow.AutoHideTab.ForeColor = Color.Red;
Please let me know if there is something else I can help you with.
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and share your thoughts.