Foreground needs a Brush, so you can use
textBlock.Foreground = Brushes.Navy;
If you want to use the color from RGB or ARGB then
textBlock.Foreground = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromArgb(100, 255, 125, 35));
or
textBlock.Foreground = new System.Windows.Media.SolidColorBrush(Colors.Navy);
To get the Color from Hex
textBlock.Foreground = new System.Windows.Media.SolidColorBrush((Color)ColorConverter.ConvertFromString("#FFDFD991"));