此图标的感叹号是透明的部分，因此一个技巧是在其后面添加背景以具有所需的颜色。当然，背景不应该覆盖整个区域，因此我们需要使用渐变来覆盖其中的一部分。

.fa-exclamation-triangle {

  background:linear-gradient(red,red) center bottom/20% 84% no-repeat;

}


<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.3.1/css/all.css">

<i class="fas fa-exclamation-triangle fa-7x"></i>

<i class="fas fa-exclamation-triangle fa-4x"></i>

<i class="fas fa-exclamation-triangle fa-2x"></i>

V4也是如此：

.fa-exclamation-triangle {

  background:linear-gradient(red,red) center /20% 70% no-repeat;

}


<link href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">

<i class="fa fa-exclamation-triangle fa-5x"></i>

<i class="fa fa-exclamation-triangle fa-4x"></i>

<i class="fa fa-exclamation-triangle fa-2x"></i>

也是SVG版本：

.fa-exclamation-triangle {

  background:linear-gradient(red,red) center bottom/20% 84% no-repeat;

}


<script defer src="https://use.fontawesome.com/releases/v5.3.1/js/all.js" ></script>

<i class="fas fa-exclamation-triangle fa-7x"></i>

<i class="fas fa-exclamation-triangle fa-4x"></i>

<i class="fas fa-exclamation-triangle fa-2x"></i>

更新

为了使答案更通用，我们还可以考虑多个背景和径向渐变，以便为任何形状着色。诀窍是在背景上覆盖透明部分而不会溢出。

这是一些图标示例：

.fa-exclamation-triangle {

  background:linear-gradient(red,red) center bottom/20% 84% no-repeat;

}

.fa-ambulance {

 background:

  linear-gradient(blue,blue) 25% 30%/32% 45% no-repeat,

  radial-gradient(green 60%,transparent 60%) 15% 100%/30% 30% no-repeat,

  radial-gradient(green 60%,transparent 60%) 85% 100%/30% 30% no-repeat;

}

.fa-check-circle {

  background:radial-gradient(yellow 60%,transparent 60%);

}

.fa-angry {

 background:

  radial-gradient(red 60%,transparent 60%) 25% 40%/30% 30% no-repeat,

  radial-gradient(red 60%,transparent 60%) 75% 40%/30% 30% no-repeat;

}


<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.3.1/css/all.css">

<i class="fas fa-exclamation-triangle fa-7x"></i>

<i class="fas fa-ambulance fa-7x"></i>

<i class="fas fa-check-circle fa-7x"></i>

<i class="fas fa-angry fa-7x"></i>