Give one spread its own background color

Two ways in. Right-click a spread in the spread timeline and choose Background Color - select several spreads first to color them all at once. Or, on the spread itself, press B (or click the background button in the toolbar) and choose "or choose a background color". Either way, pick a color, use Pick from page to sample one from your photos, or type RGB values. Use for all spreads makes the color the project background color, and Reset to project color puts the spread back on the project color.