Couple ways you could do this. I would not recommend adding in purple or blue, it will just start making things confusing unless you really know what you\'re doing.
For a more natural looking yellow, go with the yellow brown for shades like obsidianpainting suggested. I like Snakebite Leather, then use something like Scorched Brown for further shades.
For a more cartoon/fantasy looking yellow shade with orange. The GW Foundation orange is a great color for this. Not too vibrant, but not too brown.
-Matt