I have a background image gradient which is 750px X 600px in size.
I have applied this to my site however I want the bottom gradient colour to show when the page extends beyond 600px in height.
The problem is that I want to include the background colour but restrain it within a width of 750px as is the same as my background image. When I go to include the background colour along with the image it extends to fill the whole screen.
I think I see the problem. You have an image with a set width AND set height. I get the impression you dont want the user seeing anything outside of the 750px width and want the background color showing if the height goes beyond 600px, is that true?
I would say see if you can create your background gradient image that can repeat horizontally then you are golden. If you really can't do this, seems like you would have two options: one is to use JavaScript to resize the browser window to 750px wide (not recomended). Option two is to nicely place your primary content in a container DIV with your background image, set the width of that to 750px and then perhaps put a nice border around the container and center it with a nicely complimented background color.
Wrap everything in a container that is set to 750px with the background image and colour that you want then you can position the container anywhere you like within your page.