The current control  isn't set up to allow that. You would have to write
your own custom control. The question is, what metadata would you key
off of for the colors? Since the NavBar is dynamic you would have to
build in some algorithm for determining which color  to make the tab.

There doesn't appear to be an ID for each tab, but if you want to use a
little client-side JavaScript to manipulate the tabs, the following
example will change the background color  of a Team Site top-link bar tab
red with white text:

<script type="text/javascript">
try
{
var topNav = document.getElementById("onetIdTopNavBarContainer");
var links = topNav.getElementsByTagName("A");

for (var i=0; i < links.length; i++)
{
var link = links[i];
if (link.parentNode.nodeName == "TD")
{
if (link.innerText == "Child Site")
{
link.style.cssText = "background-image: none; background-color:
red; color: white;";
}
}
}
}
catch(e)
{
alert(e.Message);
}
</script>