radial-gradient()
function can be used creates a circular (or ellipis) background image of color stops. There are a ton of options around the location of the origin, colors, etc.
Code
<div class="box"></div>
.box {
height: 500px;
width: 500px;
background-image: radial-gradient(
circle at center,
red,
orange,
yellow,
green,
blue,
indigo,
violet
);
}