There is no simple way to do this in the Portal. You got to open the .css files SPS.CSS, OWS.CSS etc. found under

C:\Program Files\Common Files\Microsoft Shared\web server extensions\60\TEMPLATE\LAYOUTS\1033\STYLES or the LAYOUTS\1033\STYLES directory of each portal and change the relevant styles as described in the link given below.

msdn.microsoft.com/.../...zingSharePointSites3.asp

for ur specific question. Changing the background-color property under WPHeader style should do the job, or better would be to store the bar  as an image  on the Images directory and specify its location in the background-image property under WPHeader.

CAUTION: Take a backup before playing around with the stylesheets cos, Also all are not background colors some r images no matter what background you so, have a colorful time

Changing the WSS sites is a lot easier, cos I decided to stick to options provided to FrontPage

WPTitle and WPHeader are the styles we needed to modify. WPTitle is the
left cell of the two-cell WP header and the one we were most interested in.
WPHeader includes the WPTitle cell and the cell for the drop-down menu.

Operationally, we:

1. Renamed sps.css to sps-ms.css; ows.sps to ows-ms.css and owspers.css to owspers-ms.css
2. Copied the specific classes we needed to modify to client-local.css and modified them there
3. Then used the following batch file to concatenate sps-local.css to each
of the MS css files:
copy sps-ms.css+client-local.css sps.css
copy ows-ms.css+client-local.css ows.css
copy owspers-ms.css+client-local.css owspers.css

This approach allowed us to:

1. Leave the MS css files intact and unchanged - just renamed.
2. Localize all of our CSS changes to a single file: client-local.css
3. From a maintenance, deployment and supportability perspective, it was a simple matter of editing client-local.css and the running the batch file to recreate sps.css, ows.css and owspers.css.

If I understand your goal, I think that your best bet will be to alter the
underlying CSS. Be prepared for a lot of trial and error. But I think that
it could be done.