Content specific properties in navigation

I am trying to create a menu that using color coding that reflects properties i on the page/listitem (its underlying content type)

I have made a new SiteMap provider that inherits from PortalSiteMapProvider, but i cannot see any obvious way to parse properties from the listitem to the resulting SiteMapNode.

Can anybody tell me how this is done the "proper" way Because i guess connecting to the list item using the URL would have a bad impact on performance.