How to change Canvas background in Flutter

Question

Why background is black how to solve my problem....

enter image description here

Future<Uint8List> _generateImageFromString(String text, TextAlign align,) async {
    ui.PictureRecorder recorder = ui.PictureRecorder();
    final canvas = Canvas(
        recorder,
        Rect.fromCenter(
          center: Offset(0, 0),
          width: 550,
          height: 400, // cheated value, will will clip it later...
        )
    );
    final span = TextSpan(
      style: const TextStyle(
        backgroundColor: Colors.white,
        color: Colors.black,
        fontSize: 20,
        fontWeight: ui.FontWeight.bold,
      ),
      text: text,
    );
    final tp = TextPainter(
        text: span,
        maxLines: 3,
        textAlign: align,
        textDirection: TextDirection.ltr
    );
    tp.layout(minWidth: 550, maxWidth: 550);
    tp.paint(canvas, const Offset(0.0, 0.0));
    var picture = recorder.endRecording();
    final pngBytes = await picture.toImage(
      tp.size.width.toInt(),
      tp.size.height.toInt() - 2, // decrease padding
    );
    final byteData = await pngBytes.toByteData(format: ui.ImageByteFormat.png);
    return byteData!.buffer.asUint8List();
  }

Installed Pluging for printing: blue_thermal_printer: ^1.2.0

How to solve my problem?