I think it's genetic. I have three Itohs directly adjacent that box Joanna Marlene in in the front of a bed and all are green while Joanna Marlene remains this distinct bronze colour in late summer. That's an interesting response that Val received as Joanna Marlene is the only Itoh I have seen turn this colour; Impossible Dream turns a deep red, Scarlet Heaven and Simply Red have the "red curse' that makes it look like they have a terrible case of measles, and all the others look like the herbaceous cultivars in fall, occasionally showing shades of yellow, red, and orange in their foliage but often remaining green.