JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/font-awesome/4.2.0/css/font-awesome.min.css">
<i class="fa fa-camera-retro rainbow"></i>

CSS

.rainbow {
  background-image: -webkit-gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) );
  background-image: gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) );
  color:transparent;
  -webkit-background-clip: text;
  background-clip: text;
}