Some effects have a "background" button, and clicking on that will give you this dialog.
You can choose to have a simple single solid color as the background or something more dramatic. In the above example I've chosen tilted shading repeating 7 times with purple and magenta as the two colors.
What is the difference between circular and elliptical? If the image is square, nothing! But if the image is not square, like a long rectangle or a tall rectangle, the elliptical shading will be "distorted" to fit inside the rectangle.
If you choose "Solid Color" then you can only select one color and the Color 2 button will be greyed out.
The Swap button would change (in the above example) Color 1 to purple and Color 2 to magenta.
When the ripple frequency is close to 0 you will see no ripples of color. As you increase the ripple frequency the shading will repeat itself.
The PRO version has a very similar dialog to create original gradient images.