You could add a span before the link with a specific class like so:
<div class="btn btn_red"><span class="icon"></span><a href="#">Crimson</a><span></span></div>
And then give that a specific width and a background image just like you are doing with the button itself.
.btn span.icon {
background: url(imgs/icon.png) no-repeat;
float: left;
width: 10px;
height: 40px;
}
I am no CSS guru but off the top of my head I think that should work.