Simple Drawable extension to allow highlighting regions of a single image. Also allows an overlay to be drawn on top of the selection (I've use a bitmap for the overlay, but using a color instead is straightforward and follows the same technique as darkening the surrounding area).

HighlightableBitmapDrawable.java

package foo.bar;

import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.Region;
import android.graphics.drawable.BitmapDrawable;
import android.support.annotation.ColorRes;
import android.support.annotation.DrawableRes;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;

/**
 * Drawable which can highlight a selection area and apply an optional overlay.
 */
public class HighlightableBitmapDrawable extends BitmapDrawable {
    private Rect mSelection;
    private Bitmap mOverlay;    // bitmap to be overlaid on top of selection area
    private final int mColorRes;

    /**
     * @param drawable The underlying image.
     * @param darkenColor The color to darken the underlying image with.
     */
    public HighlightableBitmapDrawable(@NonNull Resources resources, @DrawableRes int drawable, @ColorRes int darkenColor) {
        super(resources, BitmapFactory.decodeResource(resources, drawable));
        mColorRes = darkenColor;
    }

    /**
     * Highlights the given region.
     * @param region The region to highlight. Can be null to clear current selection.
     */
    public void selectRegion(@Nullable Rect region) {
        mSelection = region;
    }

    public void selectRegion(int left, int top, int right, int bottom) {
        selectRegion(new Rect(left, top, right, bottom));
    }

    public void clearSelection() {
        selectRegion(null);
    }

    /**
     * Sets an overlay image to be drawn inside the selection bounds.
     * @param overlay The bitmap to use. Can be null to remove current overlay.
     */
    public void setOverlay(@Nullable Bitmap overlay) {
        mOverlay = overlay;
    }

    public void setOverlay(@NonNull Resources resources, @DrawableRes int overlay) {
        setOverlay(BitmapFactory.decodeResource(resources, overlay));
    }

    public void clearOverlay() {
        setOverlay(null);
    }

    @Override
    public void draw(Canvas canvas) {
        super.draw(canvas); // Draw the underlying image.
        if (null != mSelection) {
            if (null != mOverlay) {
                // If we have an overlay, paint it on top of the selected area.
                //  (This must be done BEFORE clipping the canvas or it won't show.)
                canvas.drawBitmap(mOverlay, null, mSelection, null);
            }
            // If we have a selection, darken its surroundings.
            canvas.clipRect(mSelection, Region.Op.DIFFERENCE);
            //noinspection ResourceAsColor
            canvas.drawColor(mColorRes);
        }
    }
}