Irises do best with at least six hours of sun. Most of them don't bloom as much if they get less. That said, I do have a shorter light yellow historic iris that blooms it's fool head off in full shade, so there are varieties out there that could fill your spot. I don't know names off hand, though, so maybe I'm not much help.

You could try an iris, even though they prefer more sun, because they are cheap. If you want a brick or rosy red or burgundy there are numerous choices, but no scarlets.

Iris Albicans (cemetery iris) bloom in shade and seem to thrive everywhere, and they are white.

I'll let someone closer to your zone recommend a good yellow or white.

By far the best white iris for shade I've found is Iris tectorum alba - the white version of Japanese roof iris. It is probably shorter than you want, and not easy to find, but is a very good perennial. Established siberians can handle a fair amount of shade, but you may be pushing it.