Hi Jeff. What a lot of editors do in this situation is to have the background match the image/footage ‘exactly’ except that it is enlarged and blurred so as not to distract.
To do this, copy the video track to below your main track, use the crop/pan to enlarge it however you want, and then add some Gaussian blur to create the effect you want.
Much neater and less imposing than a continuously colour-changing background to match your images.
Try it, it’s very simple and effective. Cheers.