grid_test.dart

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';

const kSwatchSize = 36.0;

void main() {
  runApp(new MaterialApp(
      home: new Scaffold(
    body: new Stack(children: [
      new Positioned(
        left: 200.0,
        top: 200.0,
        child: new SizedBox(
            width: 600.0,
            height: 300.0,
            child: new GridView.count(
              crossAxisCount: 6,
              children: _getColorMenuTileItems(),
            )),
      ),
    ]),
  )));
}

List<PopupMenuItem<Color>> _getColorMenuTileItems() {
  final colors = Colors.primaries.map((c) => c).toList();
  colors.addAll([Colors.black, Colors.grey, Colors.white]);
  return colors.map((c) {
    return new PopupMenuItem<Color>(
      value: c,
      child: new GridTile(
          child: new Container(
        width: kSwatchSize,
        height: kSwatchSize,
        color: c,
      )),
    );
  }).toList();
}