def nucleotide_colors_to_fornac_color_string(nucleotide_colors):
    """
    Convert a dictionary of per nucleotide colors to a fornac
    color string.

    :param nucleotide_colors: A dictionary with nucleotide numbers as keys and colors as values.
                              (e.g. {1: (255,0,0), 2: (255,255,0)})
    :return: A color string (e.g "1:rgb(255,0,0) 2:rgb(255,0,0)")
    """
    color_string = ''
    for key in nucleotide_colors:
        color_string += '{}:rgb({},{},{}) '.format(key, nucleotide_colors[
            key][0], nucleotide_colors[key][1], nucleotide_colors[key][2])
    return color_string
