Are the flagstones flush? Or is there a gap between next stone? If there’s a gap you could put in tight ground cover, or Johnny Jump Ups. If pretty flush, add a colorful outdoor carpet/rug, add colorful pots with plants, fairy lights around perimeter.
With a white patio, your color options are endless! Introduce bold shots of color, or stick with white planters and colorful plants. Use primary colors for poly furniture, or black wrought iron, or sling style furniture in bolder shades, with an umbrella to match, supplemented with lighting under the umbrella canopy.
A pergola, large tilting umbrella, or retractible awning is recommended if sun is intense in your area. If you want further protection from the elements, consider enclosing a portion of the space.
You realize that the larger the space, the more potential for multiple seating areas and many more plants. If the space permits, you can place an oversized planter at mid point, with a large potted tree, and seating around that space. Make sure you plan for side tables that won't tip over on the flagstones.
Size? Photo? The obvious answer is to install flower pots and furniture.
you could use an outdoor rug with some color in it and here are some other ideas
12 Patio Decorating Ideas for Spring and Summer | HGTV
https://www.hgtv.com › ... › Outdoor Spaces › Decks, Porches and Patios
Lots of colorful flowers everywhere