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 });