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),
      ),
    );
  }
}