Nathaniel, The bag should be the same color as the hood and the bumper. The bag in your picture looks tan to me. If it is, the origial yellow one must have been replaced along the way.
It seems to me when in North Canton last year, one of the Hoover gurus said that model came with both colors of bag, something Hoover would do midstream during production. I have a Citation from the 50's, and have seen 3 variations of original bags on that model, so would not surprise me at all. Very nice vac btw...