just need random color package

slow_scroll_flutter.dart

import 'dart:async';

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

class SlowScrollView extends StatefulWidget {
  @override
  _SlowScrollViewState createState() => _SlowScrollViewState();
}

class _SlowScrollViewState extends State<SlowScrollView>
    with SingleTickerProviderStateMixin {
  RandomColor _randomColor = RandomColor();
  Timer _timer;
  double _top = -150;
  int _count = 100;
  int _cross = 50;

  List<List<Color>> _colors = [];
  final w = 150.0;
  final h = 250.0;
  final _spacing = 10.0;
  @override
  void initState() {
    _setUp();
    _timer = Timer.periodic(
      Duration(milliseconds: 200),
      (_) {
        if (mounted)
          setState(() {
            _top -= 1;
            if (_top == (h + (_spacing * 2)) * _count) {
              _setUp(add: 100);
            }
          });
      },
    );
    super.initState();
  }

  void _setUp({int add = 0}) {
    for (int c = 0; c < _cross; c++) {
      _colors.add([]);
      for (int i = 0; i < (_count + add); i++) {
        _colors[c].add(_randomColor.randomColor());
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Stack(
        children: <Widget>[
          for (int i = 0; i < _cross; i++)
            ..._buildColumn(i, (i * w) + _spacing),
        ],
      ),
    );
  }

  List<Widget> _buildColumn(int c, double left) {
    final _children = <Widget>[];
    double _offsetX = 0;
    double _offsetY = 0;
    if (c.isOdd) {
      _offsetX = h / 2;
    }
    _offsetY = 0 - w / 2;
    for (int i = 0; i < _count; i++) {
      _children.add(AnimatedPositioned(
        top: _offsetX + (_top + (i * h)),
        left: _offsetY + left,
        duration: Duration(milliseconds: 50),
        child: Container(
          width: w - _spacing,
          height: h - _spacing,
          color: _colors[c][i],
        ),
      ));
    }
    return _children;
  }
}