To set the background color for the rect SVG element, you can use the fill attribute on the rect element inside the svg tag in HTML.
TL;DR
<!-- Set background color on 'rect' svg element -->
<svg>
<rect x="0" y="0" width="30" height="20" fill="red" />
</svg>
For example to set the color red on the rect svg element, let's use the fill attribute and set its value to the string red like this,
<!-- Set background color on 'rect' svg element -->
<svg>
<rect fill="red" />
</svg>
But this alone wouldn't show anything on the screen as the browser doesn't know where to start drawing the shape. For that, you also have to add other 4 attributes like the x, y, width, and height. To know more about these attributes check out How to create or draw a rectangle using SVG in HTML?
So at the end it will look like this,
<!-- Set background color on 'rect' svg element -->
<svg>
<rect x="0" y="0" width="30" height="20" fill="red" />
</svg>
See the above code live in JSBin.
That's all 😃!