Here are some pics comparing the visor bought from Virginia Mustang (ivy gold) and the original visors and headliner. As you can see the new visor is darker:
I'm not sure how long ago you purchased these, but the problem may be the simple fact of how the manufacturer "names" the colors. I don't want to confuse anyone, but here it goes...
The manufacturer may, does, and has used the name "ivy gold" on a material that is not the ivy gold that came in your car. The real color that you need, as Murf pointed out, is called "light green." (Unless, as Charles pointed out, the car came with the off white color-which is just called "white.")
It looks as if you may have gotten "Ivy Gold." That is definitely too dark. We don't even stock that color for a 65-66 right now. We also do not even use the name "ivy gold" in the catalog. If we once did, we don't any more.
I know this is a bit confusing, but hopefully I explained it so it makes some sense.
So, again, just order light green or white. Light Green should be really, really close (if that is what you have and not white) ....and don't even get me started on the carpet color names for the same thing, but the manufacturer calls it Moss Green.