echo "<u><font color='red'><br>$username</font></u>";

text-decoration: none;
border-bottom: 1px solid #000";

<style>
.redUnderline {
    color: #ff0000;
    border-bottom: 1px solid #000000;
}
</style>
<span class="redUnderline">$username</span>

<style>
a.blackUnderline {
   color: #000000;
   text-decoration: underline;
}
.red {
    color: #ff0000;
}
</style>
<a href="" class="blackUnderline"><span class="red">$username</span></a>

<style>
u { color: black; }
.red { color: red }
</style>
<u><span class='red'><br>$username</span></u>