How can I only change the background color of the very 1st page in Pages?

Hi Marine,

Insert a Section Break at the bottom of Page 1.

Insert a rectangle shape, set the colour and resize the rectangle to fill the whole of page 1.

With the rectangle selected, Menu > Arrange > Section Masters > Move Object To Section Master.

It will appear "behind" the page and will be confined to that Section (Page 1).

If you should need to edit the rectangle, Menu > Arrange > Section Masters > Make Master Objects Selectable.

Regards,

Ian.