def split_into_formatters(compound):
    """Split a possibly compound format string into segments.

    >>> split_into_formatters('bold_underline_bright_blue_on_red')
    ['bold', 'underline', 'bright_blue', 'on_red']

    """
    merged_segs = []
    # These occur only as prefixes, so they can always be merged:
    mergeable_prefixes = ['on', 'bright', 'on_bright']
    for s in compound.split('_'):
        if merged_segs and merged_segs[-1] in mergeable_prefixes:
            merged_segs[-1] += '_' + s
        else:
            merged_segs.append(s)
    return merged_segs