Ah, you want the colour to alternate.
The easiest way would be to use a 1x1 animated gif which flashes red, blue and white and then use this as the background istead of yellow. I don't have the tools to create this however, but here is the code you would need to use:
HTML Code:
<if condition="$show['member']">
<td <if condition="$bbuserinfo[pmunread]">style="background: url('path/to/image.gif')"<else />class="alt2"</if> nowrap="nowrap">
<div class="smallfont">
<strong><phrase 1="$bbuserinfo[username]" 2="member.php?$session[sessionurl]u=$bbuserinfo[userid]">$vbphrase[welcome_x_link_y]</phrase></strong><br />
Again, you or somebody else would have to provide the image, and you would have to upload this to your webserver and edit where it says 'path/to/image.gif' to the location of your image yourself, but this is the simplest way.