I want to fill blank background in colour while using pages

Nothing to stop you using colour fill - open the Inspector in iWeb and then go to Page Layout and then click on either the Page background or browser background and choose either colour fill or image fill or gradient fill and that's it.

You've asked in the forum for iWeb (the website building program) and so, not surprisingly, you are getting answers applicable to that.

If you would like to ask about Pages then the best place to ask is in the Pages Forum.

As far as I know in Pages you woiuld have to place a blank shape on the page, resize it to fill the page, add the color fill you want to it to have and move it to the back.