Not sure if everybody who suggested it is aware, but cigarette smoke really stinks. If you're looking for a stinky pickguard, though, this approach will definitely get you there. Slowly.
Tea or coffee sounds like a good idea, and they will get you there, I guess. Slowly.
RIT dye = fast + easy , which is for me, because wvbass = lazy!
Not sure how well these pictures capture it but here is the pickguard I did:
The jazz bass has its original, unmodified pickguard. The precision has been through a RIT treatment. I wanted a subtle change, which I think I got. When I first dyed it, I really overdid it. I used the steel wool to remove some of the discoloration. The steel wool is very important. The dye won't stick at all to the smooth pickguard.
As you can see, it still looks like a white pickguard, but it doesn't look like its going to glow in the dark anymore. It is also easy to change the color very significantly, if you choose. The pictures of the guitar that are linked to in this thread look to be a lot more dramatic a color change than I wanted. I also used tan instead of yellow because I wanted a darker, duller color.
(If you look closely, you can see the grill of my Baby Blue reflected in the j bass pickguard. The steel wooled and dyed guard won't act like a mirror, which also cuts down on its perceived "brightness.")