If I'm understanding you correctly you want to render on a plain bg color other than grey correct? If this is what you are saying ...

ON the bottom right hand of your model there's the four round buttons.... (Pink one in the image above is the bg button)... that's how I change the bg color... when I got to render I set it at "Render over BG color" and it works for me... did I help or was this post completely pointless... Runs back to corner