Transparent movie - background color (default is black)

When i turn off world visibility for camera, and set movie rendering to transparent, the default background color is black. Can i change it? I do want clear single color background just like the default black, but i want it to be grey for my purposes.

Can i do it somehow without publishing transparent png sequence images and then putting it together with a solid background?

I have world set to be an image that casts nice reflections on my object, but i want the object to be rendered on solid background with no reflections or light whatsoever. When i turn off world visibility for camera its perfect, just need to change the black to grey.

For a transparent background you need to render to a movie format that actually supports transparency (most do not).
There is quicktime animation codec or quicktime png

If you just want a coloured background then in the compositor you can render to a transparent background and use an alpha over node to put over a plain RGB input node