Font Awesome Background color

you can use fa-stack with the fa-circle icon under the other to have better control on the pixel just at the border:

<span class="fa-stack">
    <i class="fa fa-circle fa-stack-1x icon-a"></i>
    <i class="fa fa-times-circle fa-stack-1x icon-b"></i>
</span>

with:

.icon-a {
    color: #FFF;
    font-size: 0.9em;
}
.icon-b {
    color: #000;
}

UPDATE: As xavier pointed out below, font-awesome has Stacked Icons which will let you put a circle behind an icon without using a hack. Essentially, you stack whichever icon you want on top of their fa-circle icon. You can then style the circle independently from the icon and change it to whatever color you'd like.

Here's an example based off of code from their site:

.fa-circle {
  color: black;
}

<link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.4/css/all.min.css" rel="stylesheet"/>

<span class="fa-stack" style="vertical-align: top;">
  <i class="fas fa-circle fa-stack-2x"></i>
  <i class="fas fa-chevron-up fa-stack-1x fa-inverse"></i>
</span>

ORIGINAL ANSWER: Unfortunately, the whole icon is considered a single "character" on the page and as such, you don't have that granular of control over it. You can simply set a single "color" property to set the color of the character/icon to be what you want.

Now, it is possible for a creative hack though. Since the middle of the character is transparent, you can set a colored background behind the character to make it appear that the middle is a different color by doing something like this:

    <i class="fa fa-chevron-circle-up"></i>

then in your CSS:

.fa-chevron-circle-up {
  background: yellow;
  border-radius: 50%;
  height: 1em;
  width: 1em;
}

If the background circle offsets the icon, you can use line-height to fix it.