I`ve found plenty of hex editors for changing sprite appearances, but I can`t find anything to change color pallettes and edit the very large images. Any suggestions? (Also, sorry if this ends up being a multi-post. My phone is having connection errors.)

I am not sure what you mean by "very large images" - are you talking about cutscenes and menus and such? If they don't show up in a graphics editor, then they are probably compressed, and the only way to edit them is to decompress them. I'm guessing it's going to get pretty messy at that point.

Ragey's site has some info on rom graphics, though the page is oriented towards ripping sprites, not editing them. Ragey might be able to give you some info on this

Also, check out Romhacking.net:

Some NES graphics editors
Some palette editors

That may be what you're looking for.