How to I change the disabled color of an ElevatedButton

You can copy paste run full code below
You can use ButtonStyle and check states.contains(MaterialState.disabled) return color you need
In demo code, disabled color is green
code snippet

    ElevatedButton(
      onPressed: null,
      child: Text('Submit disable'),
      style: ButtonStyle(
        backgroundColor: MaterialStateProperty.resolveWith<Color>(
          (Set<MaterialState> states) {
            if (states.contains(MaterialState.pressed))
              return Theme.of(context)
                  .colorScheme
                  .primary
                  .withOpacity(0.5);
            else if (states.contains(MaterialState.disabled))
              return Colors.green;
            return null; // Use the component's default.
          },
        ),
      ),
    ),

working demo

full code

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            ElevatedButton(
              onPressed: null,
              child: Text('Submit disable'),
              style: ButtonStyle(
                backgroundColor: MaterialStateProperty.resolveWith<Color>(
                  (Set<MaterialState> states) {
                    if (states.contains(MaterialState.pressed))
                      return Theme.of(context)
                          .colorScheme
                          .primary
                          .withOpacity(0.5);
                    else if (states.contains(MaterialState.disabled))
                      return Colors.green;
                    return null; // Use the component's default.
                  },
                ),
              ),
            ),
            ElevatedButton(
              onPressed: () {
                print("clicked");
              },
              child: Text('Submit enable'),
              style: ButtonStyle(
                backgroundColor: MaterialStateProperty.resolveWith<Color>(
                  (Set<MaterialState> states) {
                    if (states.contains(MaterialState.pressed))
                      return Theme.of(context)
                          .colorScheme
                          .primary
                          .withOpacity(0.5);
                    else if (states.contains(MaterialState.disabled))
                      return Colors.green;
                    return null; // Use the component's default./ Use the component's default.
                  },
                ),
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}