Have you looked on the colour wheel for a best shade for your particular yellow?
I know its a really \'gimpy\' thing to do and you might think its a waste of time but its always a good idea when your stuck to consult the font of knowledge that is a colour wheel.
I personally choose a liquid orange or brown, not an ink, more of a glaze, which you can recreate by mixing about 5ml of light brown ink (or 2ml of dark brown) with about 15ml (or 18) of Future Floor Finish/Klear or any other acrylic medium that is suitably fluid. I find this will give the FFF just a tint of brown, which allows you to build up the shadows gradually by applying more and more glazes of brown.
You could also try a medium orange for a more \'sun\' yellow, or go for ochres or green-grey-browns for more muted colour yellows. I\'ll whip out a copy of Coreldraw or something and try to get some colour chips on the sorts of colours I\'m thinkin\' of, gimmie five mins and I\'ll see what I can come up with.
If you can let me know what type of yellow you want to shade that would be a great help too?