Coloured Glow

HTML

<svg id="glowy" width="400" height="400" viewBox="0 0 400 400" xmlns="http://www.w3.org/2000/svg" version="1.1">
    
    <defs>
      <!-- We set the filter effect area to be really big to guarantee we see it all... -->
      <filter id="white-glow" x="-5000%" y="-5000%" width="10000%" height="10000%">
          <feFlood result="flood" flood-color="#ffffff" flood-opacity="1"></feFlood>
          <feComposite in="flood" result="mask" in2="SourceGraphic" operator="in"></feComposite>
          <feMorphology in="mask" result="dilated" operator="dilate" radius="2"></feMorphology>
          <feGaussianBlur in="dilated" result="blurred" stdDeviation="5"></feGaussianBlur>
          <feMerge>
              <feMergeNode in="blurred"></feMergeNode>
              <feMergeNode in="SourceGraphic"></feMergeNode>
          </feMerge>
      </filter>

      <!-- The only difference in this filter is its "flood-color" -->
      <filter id="purple-glow" x="-5000%" y="-5000%" width="10000%" height="10000%">
          <feFlood result="flood" flood-color="#7d26cd" flood-opacity="1"></feFlood>
          <feComposite in="flood" result="mask" in2="SourceGraphic" operator="in"></feComposite>
          <feMorphology in="mask" result="dilated" operator="dilate" radius="2"></feMorphology>
          <feGaussianBlur in="dilated" result="blurred" stdDeviation="5"></feGaussianBlur>
          <feMerge>
              <feMergeNode in="blurred"></feMergeNode>
              <feMergeNode in="SourceGraphic"></feMergeNode>
          </feMerge>
      </filter>
        
    </defs>
    
    <g transform="translate(20,20)">
        <rect x="0" y="0"  width="40" height="40" rx="5" ry="5" style="fill: #ddd; filter:url(#white-glow)"/>
        <rect x="0" y="50"  width="40" height="40" rx="5" ry="5" style="fill: #d00; filter:url(#purple-glow)"/>
        
        <path transform="translate(90,20)" d="M0,0L10,10L20,0L30,10" stroke="black"
              filter="url(#white-glow)" stroke-width="3" fill="none"/>
  ...

CSS

body { background-color:#333; }