I think all you'd need to do is edit the public_forum_summary.pl and public_forum_page.pl templates to not alternate the colours.
In 6.05 theres bits that do something along the lines of:
if ($alt_color eq "$vars_style{AltColumnColor1}") {
$alt_color = "$vars_style{AltColumnColor2}";
} else {
$alt_color = "$vars_style{AltColumnColor1}";
}
just comment these out with preceding each line with a hash (#).
Then just set your 2nd alternating column colour in the CP.
I'm sure it's not all that different in the later ubbs, but of course I could be wrong
