Hello grahame mcgirr,
When a ToolWindow is hidden and then you show it, the DockStateChanging/DockStateChanged events are fired. This is the place where you can change the appearance of RadCalendar. Please refer to the following code snippet:
sender, DockStateChangingEventArgs e)
(e.DockWindow.DockState == DockState.Hidden)
// do sth with RadCalendar
The easiest option for you to change the color of specific dates is to add these dates in the RadCalendar.SpecialDays collection:
This will change the color of the date cells to the color defined by the theme.
I hope this helps. If you have additional questions, feel free to contact me.
the Telerik team