When you have an SVG image on your site and need to transition the fill color on hover, for instance, you need to set the starting fill in CSS as well as the SVG itself.

The following will transition the fill color over 0.4 seconds on hover.

svg {
    fill: #2d2d2d; /* we need to also set the initial color here even
    thought it is set in the SVG itself */
    transition: all 0.4s;
}
svg:hover {
    fill: red;
}