Aperture help needed! I'm still learning...

I did a photo shoot on Monday...included a dozen portraits. One individual who could NOT be at the photo shoot has an existing portrait, and they're asking if I can edit the image to look something like (at least in the ballpark) the portraits I took:

• My portraits have a gray/blue backdrop.

• The provided photo has a bright white (like a wall!) backdrop.

I'm still learning the ins and outs of Aperture...and I cannot figure out how to change the white background to something close to what I used for the other shots. I've tried the color/tint brush features, and nothing's happening. I've looked at a couple of Aperture video tutorials on the Apple website, and I still feel like a COMPLETE IDIOT. And I say that as someone who is fairly software savvy. I simply am unable to paint in a blue/gray color in the background.

Any help you can provide would be appreciated.

Thanks!

RB