Demonstrates how ImplicitlyAnimatedWidgets (might) work

implicit_color.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

final colors = [Colors.red, Colors.blue];

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  int index = 0;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Center(
        child: GestureDetector(
          onTap: () => setState(() => index = index == 0 ? 1 : 0),
          child: ImplicitWidget(color: colors[index]),
        ),
      ),
    );
  }
}

class ImplicitWidget extends StatefulWidget {
  const ImplicitWidget({super.key, required this.color, this.curve});

  final Color color;
  final Curve? curve;

  @override
  State<ImplicitWidget> createState() => _ImplicitWidgetState();
}

class _ImplicitWidgetState extends State<ImplicitWidget>
    with SingleTickerProviderStateMixin {
  late Color color;
  Color? startOfAnimationColor;
  late AnimationController controller;

  @override
  void initState() {
    color = widget.color;

    controller =
        AnimationController(vsync: this, duration: const Duration(seconds: 1));
    super.initState();
  }

  @override
  void didUpdateWidget(covariant ImplicitWidget oldWidget) {
    if (oldWidget.color != widget.color) {
      _animateColor(widget.color);
    }
    super.didUpdateWidget(oldWidget);
  }

  void _animateColor(Color newColor) {
    startOfAnimationColor = color;
    Animation tween = Tween<double>(begin: 0, end: 1).animate(
      CurvedAnimation(
        parent: controller,
        curve: widget.curve ?? Curves.linear,
      ),
    );
    
    tween.addListener(() {
      print(widget.color);
      setState(() {
        color = Color.lerp(
          startOfAnimationColor,
          newColor,
          tween.value,
        )!;
      }); 
    });
    controller..reset()..forward();
  }

  @override
  Widget build(BuildContext context) {
    return Container(height: 100, width: 100, color: color);
  }
}