You must be talking about the navigation bar. If somebody is on 'home' you want this field to remain black until the person goes somewhere else, right?
I am sure i've seen that before somewhere, not sure though whether it was CSS though.
However, i've seen a similar CSS where when you're stuck on one page an ever so slight border is added to the right of the link... i'll try to find that page and will post the link.
actuallt check out the top navigation bar of www.rtl.lu
, if they manage to give it 5 different colours (each related to the colour scheme of the correspondent page) you should be able to do this with the one colour only too...