Android - background color alla cell di una grid

[whisher]

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.GridView;

public class ListViewA extends Activity {
    GridView MyGrid;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        MyGrid = (GridView) findViewById(R.id.gridview);
        MyGrid.setAdapter(new ImageAdapter(this));
        // MyGrid.invalidateViews();
    }

    public class ImageAdapter extends BaseAdapter {
        Context MyContext;

        public ImageAdapter(Context _MyContext) {
            MyContext = _MyContext;
        }

        @Override
        public int getCount() {
            return 20;
        }

        public View getView(int position, View convertView, ViewGroup parent) {
            View view = convertView;

            if (convertView == null) {

                LayoutInflater li = getLayoutInflater();
                view = li.inflate(R.layout.main, null);
            }

            if (position % 2 == 0)
                view.setBackgroundColor(0x30FF0000);
            else
                view.setBackgroundColor(0x300000FF);
            return view;

        }

        @Override
        public Object getItem(int arg0) {
            // TODO Auto-generated method stub
            return null;
        }

        @Override
        public long getItemId(int arg0) {
            // TODO Auto-generated method stub
            return 0;
        }
    }

} 

main.xml

<?xml version="1.0" encoding="utf-8"?>
<GridView xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/gridview"
     android:layout_width="fill_parent"
   android:layout_height="fill_parent"
    android:padding="5dp"
    android:verticalSpacing="5dp"
    android:horizontalSpacing="20dp"
    android:numColumns="4"
    android:columnWidth="30dp"
    android:stretchMode="columnWidth"
    android:gravity="center_horizontal"
    android:background="#ffffff"
/>