Hi NorfolkLiz:
I'm surprised no one has mentioned this before now. I did not see your post last year, or I would have hollered out to you then.
You have a grafted plant - your 'Diane' Witch-hazel (Hamamelis x intermedia 'Diane'). 'Diane' has the red flowers that you have shown; I have a pretty large one in the yard off my deck here at the Valley, along with about a dozen other selections and species.
A grafted plant means that the understock (root system) is not the same (identical) plant as the scion that was grafted onto it. You are experiencing those yellow flowers on the suckers from the understock - which is definitely Hamamelis virginiana since it is flowering now in the fall. You should be able to observe closely where the yellow blooms are now, and where the red blooms were back in the spring (and will be again this coming spring). They are on entirely different branches.
If you want to continue to enjoy the red flowers of 'Diane' and want it to grow to a healthy full life, you will need to start pruning off those understock suckers - and continue to do so for the lifetime of your 'Diane'. If you allow the suckering understock to keep growing, it will outcompete the grafted portion - and eventually starve it in favor of its own existence.