[greigeh]

Use advice given above, but in your header template you should put something more like

<if condition="$bbuserinfo[fieldFIELDIDFORCOLOR]">
.tcat {
background: $bbuserinfo[fieldFIELDIDFORCOLOR];
}
</if>

So if the user has filled out that profile field, it will display their own customisation, where as if they haven't it will show the style that you have set. Simply using the $bbuserinfo[fieldFIELDIDFORCOLOR] will display blank if a user has not edited that field.