Pick main color from picture

I probably think you got a fix but for future searches to this question, I suggest you check Pallete Generator by the flutter team. I will try and give a simple explanation of how the code works but for a detailed example head over to the plugin's GitHub repo.

The example below is going to take an image then select the dominant colors from it and then display the colors

First, we add the required imports

import 'package:palette_generator/palette_generator.dart';

After that let's create the main application class.

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      ...
      home: const HomePage(
        title: 'Colors from image',
        image: AssetImage('assets/images/artwork_default.png',),
        imageSize: Size(256.0, 170.0),
      ...

      ),
    );
  }
}

In the image field above, place the image that you want to extract the dominant colors from, i used the image shown here.

Next, we create the HomePage class

@immutable
class HomePage extends StatefulWidget {
  /// Creates the home page.
  const HomePage({
    Key key,
    this.title,
    this.image,
    this.imageSize,
  }) : super(key: key);

  final String title; //App title
  final ImageProvider image; //Image provider to load the colors from
  final Size imageSize; //Image dimensions

  @override
  _HomePageState createState() {
    return _HomePageState();
  }
}

Lets create the _HomePageState too

class _HomePageState extends State<HomePage> {
  Rect region;
  PaletteGenerator paletteGenerator;

  final GlobalKey imageKey = GlobalKey();

  @override
  void initState() {
    super.initState();
    region = Offset.zero & widget.imageSize;
    _updatePaletteGenerator(region);
  }

  Future<void> _updatePaletteGenerator(Rect newRegion) async {
    paletteGenerator = await PaletteGenerator.fromImageProvider(
      widget.image,
      size: widget.imageSize,
      region: newRegion,
      maximumColorCount: 20,
    );
    setState(() {});
  }


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: _kBackgroundColor,
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Column(
        mainAxisSize: MainAxisSize.max,
        mainAxisAlignment: MainAxisAlignment.start,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: <Widget>[
          new AspectRatio(
            aspectRatio: 15 / 15,
            child: Image(
              key: imageKey,
              image: widget.image,
            ),
          ),
          Expanded(child: Swatches(generator: paletteGenerator)),
        ],
      ),
    );
  }
}

The code above just lays out the image and the Swatches which is a class defined below. In initState, we first select a region which the colors will be derived from which in our case is the whole image.

After that we create a class Swatches which receives a PalleteGenerator and draws the swatches for it.

class Swatches extends StatelessWidget {

  const Swatches({Key key, this.generator}) : super(key: key);

  // The PaletteGenerator that contains all of the swatches that we're going
  // to display.
  final PaletteGenerator generator;

  @override
  Widget build(BuildContext context) {
    final List<Widget> swatches = <Widget>[];
    //The generator field can be null, if so, we return an empty container
    if (generator == null || generator.colors.isEmpty) {
      return Container();
    }
    //Loop through the colors in the PaletteGenerator and add them to the list of swatches above
    for (Color color in generator.colors) {
      swatches.add(PaletteSwatch(color: color));
    }
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      mainAxisSize: MainAxisSize.min,
      crossAxisAlignment: CrossAxisAlignment.center,
      children: <Widget>[
        //All the colors,
        Wrap(
          children: swatches,
        ),
        //The colors with ranking
        Container(height: 30.0),
        PaletteSwatch(label: 'Dominant', color: generator.dominantColor?.color),
        PaletteSwatch(
            label: 'Light Vibrant', color: generator.lightVibrantColor?.color),
        PaletteSwatch(label: 'Vibrant', color: generator.vibrantColor?.color),
        PaletteSwatch(
            label: 'Dark Vibrant', color: generator.darkVibrantColor?.color),
        PaletteSwatch(
            label: 'Light Muted', color: generator.lightMutedColor?.color),
        PaletteSwatch(label: 'Muted', color: generator.mutedColor?.color),
        PaletteSwatch(
            label: 'Dark Muted', color: generator.darkMutedColor?.color),
      ],
    );
  }
}

After that lets create a PaletteSwatch class. A palette swatch is just a square of color with an optional label

@immutable
class PaletteSwatch extends StatelessWidget {
  // Creates a PaletteSwatch.
  //
  // If the [color] argument is omitted, then the swatch will show a
  // placeholder instead, to indicate that there is no color.
  const PaletteSwatch({
    Key key,
    this.color,
    this.label,
  }) : super(key: key);

  // The color of the swatch. May be null.
  final Color color;

  // The optional label to display next to the swatch.
  final String label;

  @override
  Widget build(BuildContext context) {
    // Compute the "distance" of the color swatch and the background color
    // so that we can put a border around those color swatches that are too
    // close to the background's saturation and lightness. We ignore hue for
    // the comparison.
    final HSLColor hslColor = HSLColor.fromColor(color ?? Colors.transparent);
    final HSLColor backgroundAsHsl = HSLColor.fromColor(_kBackgroundColor);
    final double colorDistance = math.sqrt(
        math.pow(hslColor.saturation - backgroundAsHsl.saturation, 2.0) +
            math.pow(hslColor.lightness - backgroundAsHsl.lightness, 2.0));

    Widget swatch = Padding(
      padding: const EdgeInsets.all(2.0),
      child: color == null
          ? const Placeholder(
              fallbackWidth: 34.0,
              fallbackHeight: 20.0,
              color: Color(0xff404040),
              strokeWidth: 2.0,
            )
          : Container(
              decoration: BoxDecoration(
                  color: color,
                  border: Border.all(
                    width: 1.0,
                    color: _kPlaceholderColor,
                    style: colorDistance < 0.2
                        ? BorderStyle.solid
                        : BorderStyle.none,
                  )),
              width: 34.0,
              height: 20.0,
            ),
    );

    if (label != null) {
      swatch = ConstrainedBox(
        constraints: const BoxConstraints(maxWidth: 130.0, minWidth: 130.0),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.start,
          children: <Widget>[
            swatch,
            Container(width: 5.0),
            Text(label),
          ],
        ),
      );
    }
    return swatch;
  }
}

Hope this helps, thank you.

Here's a simple function which returns the dominant color given an ImageProvider. This shows the basic usage of Palette Generator without all the boilerplate.

import 'package:palette_generator/palette_generator.dart';

// Calculate dominant color from ImageProvider
Future<Color> getImagePalette (ImageProvider imageProvider) async {
  final PaletteGenerator paletteGenerator = await PaletteGenerator
      .fromImageProvider(imageProvider);
  return paletteGenerator.dominantColor.color;
}

Then use FutureBuilder on the output to build a Widget.