The navbar-toggler-icon (hamburger) in Bootstrap 4 uses an SVG background-image.

if you want to change the color of the toggler image to something else, you can customize the icon. For example, here I set the RGB value to pink (255,102,203). Notice the stroke='rgba(255,102,203, 0.5)' value in the SVG data:
Code:
 
.custom-toggler .navbar-toggler-icon {
  background-image: url("data:image/svg+xml;charset=utf8,%3Csvg viewBox='0 0 32 32' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath stroke='rgba(255,102,203, 0.5)' stroke-width='2' stroke-linecap='round' stroke-miterlimit='10' d='M4 8h24M4 16h24M4 24h24'/%3E%3C/svg%3E");
}
.custom-toggler.navbar-toggler {
  border-color: rgb(255,102,203);
}
Here is the demo code:
Code:
<nav class="navbar navbar-dark bg-dark">
    <a href="/" class="navbar-brand">Custom</a>
    <button class="navbar-toggler ml-auto custom-toggler" type="button" data-toggle="collapse" data-target="#collapsingNavbar4">
        <span class="navbar-toggler-icon"></span>
    </button>
    <div class="navbar-collapse collapse" id="collapsingNavbar4">
        <ul class="navbar-nav">
            <li class="nav-item active">
                <a class="nav-link" href="#">Link</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Link</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Link</a>
            </li>
        </ul>
    </div>
</nav>