It's very achievable. For experiment sake, create a custom Profile Field with a single line textarea entry. Set the title to something like "Custom Username Colors". In the Description say something like "Custom user colors: Only HEX values allowed." Set max length to 7 and display size to 11. Enable it so that users can edit it. After setting the other definitions click "Save". Remember the field's ID.
In your postbit or posbit_legacy find:
<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]">$post[musername]</a>
And replace it with:
<if condition="$post[field19]"><a class="usercolor" href="member.php?$session[sessionurl]u=$post[userid]">
<span style="color:$post[field19]">$post[musername]</span></a>
<else />
<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]">$post[musername]</a>
</if>
Change the "x" for the field's ID and you're set.
Then in Main CSS add this to Additional CSS Definitions:
.usercolor
{
font-size:15px;
font-weight:bold;
text-decoration:none;
}
The underline, as far as I know, need to be set to "none" unless you remove the overall link CSS values in <body>. The reason is that the underline color will inherit the overall link color and not the custom font color.
Anyway, it should work for IE, Firefox...