Zale lunata vs. Zale minerea
Zale lunata and Zale minerea are common widespread moths in the United States and Canada, and the two are often confused. Both have multiple phenotypes, and the variation in coloration within each species is far greater than color differences between them. Here is a comparison of the forms from the Northeast:
You'll notice that lunata is consistently larger than minerea, but this doesn't help much when identifying photos of individual moths. The palps are also different in length, but the easiest way to tell them apart is by looking for the "double lobe" in the PM line which is present in lunata but absent in minerea. Here's an image showing this difference highlighted:
Of course, there are dozens of other Zales out there too, so one can't rule out every other Zale species with this rule. In particular, galbanata looks like a paler version of minerea in some of its forms, and it often gets confused with minerea (though you should never confuse it with lunata, as it lacks the bilobed PM line.) But if you're fairly certain you've got a minerea/lunata, this is a pretty consistent way to figure which one it is.
