There aren't any properties that let you set a different color for different letters in the WatermarkText, the WatermarkTextColor is used for the entire word.
If this is your goal, you can put a separate Label outside the RadEntry for the red asterisk, and show/hide that depending on if there's any Text in the RadEntry.
sender, TextChangedEventArgs e)
AsteriskLabel.IsVisible = e.NewTextValue.Length == 0;
Lance | Technical Support Engineer, Principal
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