I know what it is!

The "Mystery Bird" was just a Goldfinch. I emailed the Illinois Ornithological Society and I got a reply saying, "The bird is indeed an American Goldfinch as you first suspected.  They have a few different looks depending on the season.

Best wishes,
Geoff Williamson for IOS".

It was very disappointing, seeing as I worked on finding out what it was for at least a month. Happy birding anyway!

-Mr. Bird