ExampleActivity.java

package your.package;

import android.app.Activity;
import android.os.Bundle;
import android.support.v7.graphics.Palette;
import com.squareup.picasso.Picasso;
import your.package.PaletteTransformation;

import static your.package.PaletteTransformation.PaletteCallback;

public class ExampleActivity extends Activity {
    
  @Override public void onCreate(Bundle icicle) {
    super.onCreate();
    //...
    Picasso.with(context)
      .load(url)
      .fit().centerCrop()
      .transform(PaletteTransformation.instance())
      .into(imageView, new PaletteCallback(imageView) {
            @Override public void onSuccess(Palette palette) {
                // TODO apply palette to text views, backgrounds, etc.
            }
      });
  }
}

PaletteTransformation.java

package your.package;

import android.graphics.Bitmap;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.support.annotation.NonNull;
import android.support.v4.util.Pools;
import android.support.v7.graphics.Palette;
import android.widget.ImageView;

import com.squareup.picasso.Callback;
import com.squareup.picasso.Picasso;
import com.squareup.picasso.Target;
import com.squareup.picasso.Transformation;

import java.lang.ref.WeakReference;
import java.util.Map;
import java.util.WeakHashMap;

/**
 * Transformation used to extract {@link Palette} information from the {@linkplain Bitmap}.
 */
public final class PaletteTransformation implements Transformation {
  private static final PaletteTransformation INSTANCE = new PaletteTransformation();
  private static final Map<Bitmap, Palette> CACHE = new WeakHashMap<Bitmap, Palette>();

  /**
   * A {@link Target} that receives {@link Palette} information in its callback.
   * @see Target
   */
  public static abstract class PaletteTarget implements Target {
    /**
     * Callback when an image has been successfully loaded.
     * Note: You must not recycle the bitmap.
     * @param palette The extracted {@linkplain Palette}
     */
    protected abstract void onBitmapLoaded(Bitmap bitmap, Picasso.LoadedFrom from, Palette palette);

    @Override public final void onBitmapLoaded(Bitmap bitmap, Picasso.LoadedFrom from) {
      final Palette palette = getPalette(bitmap);
      onBitmapLoaded(bitmap, from, palette);
    }
  }

  public static Palette getPalette(Bitmap bitmap) {
      return CACHE.get(bitmap);
  }

  /**
   * A {@link Callback} that receives {@link Palette} information in its callback.
   * @see Callback
   */
  public static abstract class PaletteCallback implements Callback {
    private WeakReference<ImageView> mImageView;

    public PaletteCallback(@NonNull ImageView imageView) {
      mImageView = new WeakReference<ImageView>(imageView);
    }

    protected abstract void onSuccess(Palette palette);

    @Override public final void onSuccess() {
      if (getImageView() == null) {
        return;
      }
      final Bitmap bitmap = ((BitmapDrawable) getImageView().getDrawable()).getBitmap(); // Ew!
      final Palette palette = getPalette(bitmap);
      onSuccess(palette);
    }

    private ImageView getImageView() {
      return mImageView.get();
    }
  }

  /**
   * Obtains a {@link PaletteTransformation} to extract {@link Palette} information.
   * @return A {@link PaletteTransformation}
   */
  public static PaletteTransformation instance() {
    return INSTANCE;
  }

  //# Transformation Contract
  @Override public final Bitmap transform(Bitmap source) {
    final Palette palette = Palette.generate(source);
    CACHE.put(source, palette);
    return source;
  }

  @Override public String key() {
    return ""; // Stable key for all requests. An unfortunate requirement.
  }

  private PaletteTransformation() { }
}