How to change AlertDialog view background color?

I want to know how could I change that grey color on my alert dialog? I have tried that:

layout.setBackgroundResource(R.color.Aqua);

It didn't work. Any ideas?

I have created AlertDialog with the following code:

public class CustomInputDialog{

    private OnDialogClickListener listener;
    private Context context;
    private String title;
    private String message;
    EditText input;
    LinearLayout layout;

    public interface OnDialogClickListener {
        void onDialogOKClick(String value);
    }

    public CustomInputDialog(String title, String message, Context context, OnDialogClickListener listener) {

        super();
        this.title = title;
        this.message = message;
        this.context = context;
        this.listener = listener;

        layout = new LinearLayout(context);
        layout.setOrientation(LinearLayout.VERTICAL);
        LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.FILL_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);
        params.setMargins(20, 10, 20, 10);

        input = new EditText(context);

        InputFilter[] filters = new InputFilter[1];
        filters[0] = new InputFilter.LengthFilter(20);
        input.setFilters(filters);

        layout.addView(input, params);

    }

    private void dialog(){

        AlertDialog.Builder builder = new AlertDialog.Builder(context);
        builder.setCancelable(true);
        builder.setView(layout);
        builder.setTitle(title);
        builder.setMessage(message); 
        builder.setInverseBackgroundForced(true);
        builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
          @Override
          public void onClick(DialogInterface dialog, int which) {
                String value = input.getText().toString();
                listener.onDialogOKClick(value);
                dialog.dismiss();
          }
        });
        builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
              @Override
              public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
              }
        });
        AlertDialog alert = builder.create();
        alert.show();
    }

}