I've just received a set of furniture touch up markers which I found on the big auction site - eb#y which look to be the business. Just got a patch of light filler to colour on a repaired fore end. I'll let you know how it turns out.
I thought they were just permanent markers but it seems there is more to them, ingredients include resin acids, rosin acide and lots of other esters etc.
Not sure if they're available over the pond, I'm in the UK but it may be worth a search for furniture touch up markers.
