flutter_svg/example/assets/w3samples/alphachannel.svg

<svg id='svg1' viewBox='0 0 100 100' xmlns='http://www.w3.org/2000/svg'>
  <circle r="32" cx="35" cy="65" fill="#F00" opacity="0.5"/>
  <circle r="32" cx="65" cy="65" fill="#0F0" opacity="0.5"/>
  <circle r="32" cx="50" cy="35" fill="#00F" opacity="0.5"/>
</svg>