The fill-opacity
attribute sets the opacity of a shapes fill color. The fill-opacity
takes a decimal number between 0 and 1. The lower the value the greater the transparenty is.
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<rect x="30" y="30" height="90" width="170"
fill="#529587"
fill-opacity="0.4"/>
</svg>
Here is a small example: