Define a function named posterize. This function expects an image and a tuple of RGB values as arguments.

def posterize(img, rgb_tuple):
    num_bins = len(rgb_tuple)
    img_mod = img.copy()
    for channel in range(3):
        channel_values = img[:,:,channel]
        for bin_index in range(num_bins):
            if bin_index == num_bins - 1:
                mask = channel_values >= rgb_tuple[bin_index]
            else:
                mask = np.logical_and(channel_values >= rgb_tuple[bin_index], channel_values < rgb_tuple[bin_index + 1])
            img_mod[mask, channel] = rgb_tuple[bin_index]
    return img_mod