As Einion said, there are dozens (?) of variations of kahki, even for to your specific request for a yellow kahki.
At the risk of sounding unhelpful, I'd recommend sitting down with tubes of various earth tones and just mixing colors to see what you come up with. I often do this, playing with colors with no intent of actually painting anything, just to see what I can discover.
As you mix kahki tones, I'd only suggest that you keep the mixes simple -- no more than maybe three colors per "base" -- and trying blacks and umbers for the shading color.
If you need a more specific answer, I can post something tonight when I'm at home.