Styjun

class _HomePageState extends State<HomePage> 

 @override
 Widget build(BuildContext context) 
 return Scaffold(
 appBar: AppBar(
 title: Text("..."),
 ),
 body: Container(
 color: Colors.green,
 child: OutlineButton(
 onPressed: () ,
 color: Colors.orange,
 highlightColor: Colors.pink,
 child: Container(
 color: Colors.yellow,
 child: Text("A"),
 ),
 shape: CircleBorder(),
 ),
 ),
 );
 

The above code gives a transparent button. How can I get an orange OutlineButton?

To modify the backgroundColor of a OutlineButton you can use a DecoratedBox and a Theme widget. At the end of this answer you'll find a quick example.

Anyway I'd still recommend simply using the FlatButton with its color attribute instead.

Wrap your OutlinedButton inside a DecoratedBox. Set the shape of your DecoratedBox to the same shape your OutlinedButton. Now you can use the color attribute of your DecoratedBox to change the color. The result will still have a small padding around the OutlinedButton. To remove this you can wrap the DecoratedBox inside a Theme in which you adjust the ButtonTheme. Inside the ButtonTheme you want to set materialTapTargetSize: MaterialTapTargetSize.shrinkWrap.

The padding is added inside Flutter, to increase the tap area around the button to a minimum size of 48x48 (source). Setting materialTapTargetSize to MaterialTapTargetSize.shrinkWrap removes this minimum size.

FlatButton example:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget 
 @override
 Widget build(BuildContext context) 
 return MaterialApp(
 home: Scaffold(
 body: Center(
 child: FlatButton(
 color: Colors.pinkAccent,
 shape: CircleBorder(),
 onPressed: () => ,
 child: Text('A'),
 ),
 ),
 ),
 );
 

OutlinedButton example:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget 
 @override
 Widget build(BuildContext context) 
 return MaterialApp(
 home: Scaffold(
 body: Center(
 child: MyButton(),
 ),
 ),
 );
 


class MyButton extends StatelessWidget 
 @override
 Widget build(BuildContext context) 
 return DecoratedBox(
 decoration:
 ShapeDecoration(shape: CircleBorder(), color: Colors.pinkAccent),
 child: Theme(
 data: Theme.of(context).copyWith(
 buttonTheme: ButtonTheme.of(context).copyWith(
 materialTapTargetSize: MaterialTapTargetSize.shrinkWrap)),
 child: OutlineButton(
 shape: CircleBorder(),
 child: Text('A'),
 onPressed: () => ,
 ),
 ),
 );
 

Here's way that you can check the background color of the button.
Remove hightlightColor, and try give some value to highlightElevation property of OutlineButton, then press it, you could see initially it loads orange color.

You could use FlatButton, and just set color there.