slider

main.dart

import 'package:flutter/material.dart';

void main(){
  runApp(new MaterialApp(
    home: new MyApp(),
  ));
}

class MyApp extends StatefulWidget {
  @override
  _State createState() => new _State();
}

//State is information of the application that can change over time or when some actions are taken.
class _State extends State<MyApp>{

  double _value = 0.0;
  int r = 100;
  int g = 100;
  int b = 100;
  void _setvalue(double value) => setState(() => {
    _value = value,
    r = value * 100 > 50 ? 200 : 100,
    g = value * 100 > 50 ? 200 : 11,
    b = value * 100 > 50 ? 200 : 100
  });

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Name here'),
      ),
      //hit Ctrl+space in intellij to know what are the options you can use in flutter widgets
      body: new Container(
        color: Color.fromRGBO(r, g, b, 0.3),
        padding: new EdgeInsets.all(32.0),
        child: new Center(
          child: new Column(
            children: <Widget>[
              new Text('Value: ${(_value * 100).round()}'),
              new Text('R: ${r}, G:${g}, B:${b}'),
              new Slider(value: _value, onChanged: _setvalue)
            ],
          ),
        ),
      ),
    );
  }
}