Chems,
I am no expert but as no one else has replied I'll tell you what I have done under instructions from swmbo! I painted some pine bedroom furniture that had a light brown varnish type of finish. I used Dulux eggshell although the colour I used was an off white (it was mixed, slightly cream). After a couple of coats, and when it was properly dry, I sanded off the white paint at the corners and other wear areas so that the brown showed through. You need to experiment until it looks right but be careful not to overdo it. Later on I made some built in wardrobes out of MDF and with these I didn't bother with any (brown) undercoat, I just sanded down to the MDF. The end result was quite pleasing. At least swmbo was pleased which is all that matters!
Mike