def color_lane_pixels(img, leftx, lefty, rightx, righty):
    """
    returns an image where the detected lane pixels are
    colored
    """
    # copy image so as not to alter the original
    pix_img = img.copy()

    # color image red where the left lane was detected
    pix_img[lefty, leftx] = [255, 0, 0]

    # blue for the right lane line
    pix_img[righty, rightx] = [0, 0, 255]

    return pix_img