Finding the dominant color of an image in an Android @drawable

In Android 5.0 Lollipop, a class was added to help extract useful colors from a Bitmap. The Palette class, found in android.support.v7.graphics, can extract the following colors:

• Vibrant
• Vibrant Dark
• Vibrant Light
• Muted
• Muted Dark
• Muted Light

This Android training page gives all the details you need to use the class (I tried it myself in Android Studio and it was very straightforward): http://developer.android.com/training/material/drawables.html#ColorExtract

To quote:

The Android Support Library r21 and above includes the Palette
class, which lets you extract prominent colors from an image. To
extract these colors, pass a Bitmap object to the Palette.generate()
static method in the background thread where you load your images. If
you can’t use that thread, call the Palette.generateAsync() method and
provide a listener instead.*

You can retrieve the prominent colors from the image using the getter
methods in the Palette class, such as Palette.getVibrantColor.

To use the Palette class in your project, add the following Gradle
dependency to your app’s module:

dependencies {
    ...
    implementation 'com.android.support:palette-v7:21.0.+'
}

Or if you’re using androidx:

implementation 'androidx.palette:palette:1.0.0'

If you need to use generateAsync(), here’s how:

Palette.generateAsync(bitmap, new Palette.PaletteAsyncListener() {
    public void onGenerated(Palette palette) {
        // Do something with colors...
    }
});

EDIT:
Since the question asks how to extract colors from a drawable resource, you’d first have to convert the drawable to a bitmap to use the technique I’ve described. Luckily, that is quite simple using BitmapFactory:

Bitmap icon = BitmapFactory.decodeResource(context.getResources(),
                                       R.drawable.icon_resource);`