Using analogous colors in a painting will help tie it together.
Analogous colors are those that are next to each other on the color wheel.
Creating a simple background of analogous colors for a still life, a portrait, a floral painting is a great way to create a background with interest and texture without having to establish a lot of detail.
Choose a complimentary color to that of the main subject and use its analogous colors to create a mottled background. This will make the subject matter pop. For example, red watermelon slices the complementary color is green. Create a background of green with its analogous colors yellow and blue.