We can't see your document so we can only guess at why the page might be coloured.

Firstly, the whole document might be coloured with a background (and you can't see it on the cover page because you have a full page graphic sitting in front of it). You can go to the Ribbon's Design Tab and find the Page Color button on the right hand end to see if there is a page background assigned.

If that doesn't change anything then you may have a coloured shape in the header or footer which is sized to fill the whole page. You will need to visit the headers/footers to see if there are selectable shapes in these.

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Andrew Lockton
Chrysalis Design, Melbourne Australia