def blue_green_mirror_env(envname):
    """
    Given a blue envname, returns its green counterpart, or vice versa.
    For other envnames that aren't blue/green participants, this returns None.
    """
    if 'blue' in envname:
        return envname.replace('blue', 'green')
    elif 'green' in envname:
        return envname.replace('green', 'blue')
    else:
        return None
