When you add color to a word document using the commands, the color will appear on all pages. To add color to only one or some pages in a word document, you need to insert a rectangle box first. Please see below for steps:
Step 1: Open the document and click the "Insert" tab from the ribbon;
Step 2: Click "Shapes" and select "Rectangle" from the drop-down shapes;
Step 3: Draw a "Rectangle" shape to cover the entire page(s) you want to change the color;
Step 4: Click on one of the rectangles and in the "Format" tab in the ribbon, click the "Shape Fill" then select a color;
Step 5: Click "Wrap Text" in the "Format" tab and select "Behind Text" from the drop-down list.
Alternatively, right-click on the shape and select the "Wrap Text" from the dialog box, then select "Behind Text".
If you want to use an image as the background, please refer to how to add an image background in word.
there has to be a more official way to do this lol