1. TextBlock textBlock = new TextBlock { Inlines = { new Run { Text = "some text: " }, new Run { Text = "some text", Foreground = Brushes.White } } }; 2. textblock.Inlines.Clear(); textblock.Inlines.Add(new Run("text")); textblock.Inlines.Add(new Run("text") { Foreground = Brushes.White });