Set background color of WPF Textbox in C# code

textBox1.Background = Brushes.Blue;
textBox1.Foreground = Brushes.Yellow;

WPF Foreground and Background is of type System.Windows.Media.Brush. You can set another color like this:

using System.Windows.Media;

textBox1.Background = Brushes.White;
textBox1.Background = new SolidColorBrush(Colors.White);
textBox1.Background = new SolidColorBrush(Color.FromArgb(0xFF, 0xFF, 0, 0));
textBox1.Background = System.Windows.SystemColors.MenuHighlightBrush;

If you want to set the background using a hex color you could do this:

var bc = new BrushConverter();

myTextBox.Background = (Brush)bc.ConvertFrom("#FFXXXXXX");

Or you could set up a SolidColorBrush resource in XAML, and then use findResource in the code-behind:

<SolidColorBrush x:Key="BrushFFXXXXXX">#FF8D8A8A</SolidColorBrush>
myTextBox.Background = (Brush)Application.Current.MainWindow.FindResource("BrushFFXXXXXX");

I take it you are creating the TextBox in XAML?

In that case, you need to give the text box a name. Then in the code-behind you can then set the Background property using a variety of brushes. The simplest of which is the SolidColorBrush:

myTextBox.Background = new SolidColorBrush(Colors.White);