Flutter color interpolation

main.dart

import 'dart:ui';

import 'package:flutter/material.dart';

Color mixColors(Color a, Color b, double t) {
  final w1 = (1 - t) * a.opacity;
  final w2 = t * b.opacity;
  final n = w1 + w2;
  final w = n > 0.000001 ? w2 / n : 0.5;

  return Color.fromARGB(
    lerpDouble(a.alpha, b.alpha, t).toInt().clamp(0, 255),
    lerpDouble(a.red, b.red, w).toInt().clamp(0, 255),
    lerpDouble(a.green, b.green, w).toInt().clamp(0, 255),
    lerpDouble(a.blue, b.blue, w).toInt().clamp(0, 255),
  );
}

class MyColorTween extends Tween<Color> {
  MyColorTween({Color begin, Color end}) : super(begin: begin, end: end);

  @override
  Color lerp(double t) => mixColors(begin, end, t);
}

class MyAnimatedContainer extends StatelessWidget {
  final Widget child;
  final Color color;
  final double width;
  final double height;
  final Duration duration;

  const MyAnimatedContainer({
    this.child,
    this.color,
    this.width,
    this.height,
    this.duration,
  });

  @override
  Widget build(BuildContext context) {
    return TweenAnimationBuilder(
      child: child,
      duration: duration,
      tween: MyColorTween(begin: color, end: color),
      builder: (context, color, child) {
        return Container(
          child: child,
          color: color,
          width: width,
          height: height,
        );
      },
    );
  }
}

//-------------------------------------------------------------

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Transparent Issue',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Colors.transparent issue'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool isColorSwap = true;

  void _incrementCounter() {
    setState(() {
      isColorSwap = !isColorSwap;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            Expanded(
                child: MyAnimatedContainer(
                    child: Center(child: Text('fade between white color')),
                    width: double.infinity,
                    duration: Duration(milliseconds: 500),
                    color: isColorSwap ? Colors.lightBlue[100] : Colors.white)),
            Expanded(
                child: MyAnimatedContainer(
                    child:
                        Center(child: Text('fade between transparent color')),
                    width: double.infinity,
                    duration: Duration(milliseconds: 500),
                    color: isColorSwap
                        ? Colors.lightBlue[100]
                        : Colors.transparent))
          ]),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'ClickMe',
        child: Icon(Icons.add),
      ),
    );
  }
}