How can I transition this font awesome (v5.0) color from white to my custom color?

I am just working on a beginner project, and when I hover over the Instagram Icon, I would like it to slowly transition to the gradient of colors from some svg code I copied off of StackOverflow. I understand how the SVG code works (generally), but cannot figure out how to make the transitions work.

Here's a snippet of necessary code, along with the css that I've tried so far.

I believe the end transition value for fill is what is causing the issue. If I change it to a different value, like "blue", or "red", the transition works fine.

.social-container {
	position: absolute;
	bottom: 0;
	color: white;
	border: 1px solid white;
	display: flex;
	width: 150px;
	height: 35px;
	justify-content: space-between;
	align-items: center;
}


svg {
	transition: fill 3s ease;
}

.instagram-logo svg * {
  transition: all 2s ease;
	fill: black;
}

.instagram-logo:hover svg * {
	fill: url(#rg);
}

/* So I noticed the issue is the .instagram-logo:hover svg * {}. When I use a solid color like "blue" for the fill value, the transition works flawlessly. But when using the url, it doesn't work at all */

<script src="https://use.fontawesome.com/releases/v5.0.8/js/all.js"></script>

<div class="instagram-logo">
  <svg width="0" height="0">
    <radialGradient id="rg" r="150%" cx="30%" cy="107%">
      <stop stop-color="#fdf497" offset="0" />
      <stop stop-color="#fdf497" offset="0.05" />
      <stop stop-color="#fd5949" offset="0.45" />
      <stop stop-color="#d6249f" offset="0.6" />
      <stop stop-color="#285AEB" offset="0.9" />
    </radialGradient>
  </svg>
  <i class="fab fa-instagram fa-2x"></i>
</div>

Solution

Browsers started to support CSS transition of <color> values, so they do accept to make the transition between your CSS fill: <color> to an other fill: <color>.

But, when you are setting this CSS fill value to the gradient, for the engine, it's now an <urlFunc> value, which eventually resolves to an <image> value*. So transitioning from a <color> to this new value is not possible.

*^{Though I'm not sure about this assertion...}

One solution though would be to animate a CSS <filter> value:

.instagram-logo{
  padding: 2px 8px;
}
.instagram-logo svg.fa-instagram path{
  fill: url(#rg);
}

/* from black to color */
.instagram-logo svg.fa-instagram{
  filter: brightness(0%) grayscale(100%);
  transition: filter 2s ease, -webkit-filter 2s ease;
}
.instagram-logo svg.fa-instagram:hover{
  filter: brightness(100%) grayscale(0%);
}

/* from white to color */
.instagram-logo.white{
  background: black;
}
.instagram-logo.white svg.fa-instagram{
  filter: brightness(0%) grayscale(100%) invert(1);
}
.instagram-logo.white svg.fa-instagram:hover{
  /*
    we need to make a new declaration
    because transitions can only be done on func values 
    so we need the same <filterFunc> at both hands of the transition
 */
  filter: brightness(100%) grayscale(0%) invert(0);
}

<script src="https://use.fontawesome.com/releases/v5.0.8/js/all.js"></script>

<div class="instagram-logo">
  <svg class="hidden" width="0" height="0" style="position:absolute; z-index:-1">
    <radialGradient id="rg" r="150%" cx="30%" cy="107%">
      <stop stop-color="#fdf497" offset="0" />
      <stop stop-color="#fdf497" offset="0.05" />
      <stop stop-color="#fd5949" offset="0.45" />
      <stop stop-color="#d6249f" offset="0.6" />
      <stop stop-color="#285AEB" offset="0.9" />
    </radialGradient>
  </svg>
  <i class="fab fa-instagram fa-2x"></i>

</div>
<div class="instagram-logo white">
  <i class="fab fa-instagram fa-2x"></i>
</div>

Note that in given example, I set the filter on the parent <svg> node, because chrome doesn't seem to support CSS filtering on inner SVG nodes without prefix yet.