Khaki in itself is not a color, it can range anywhere from beige to dark olive green, its like saying "green"...or "brown".
You need to check your references and find a federal standard number (or ANA, RLM etc.) Once you get a proper color number you can find equivalency charts over the internet and some include cross-reference charts and mixing information.
Dont rely on instruction sheets that merely call that color Khaki. Olive drab is another example. The olive drab of a paint manufacturer can differ from anothers and many kit instructions are imprecise in their color callouts and often the round corners in that respect. To get accurate results, a color number must be found through research and reference matérials or through helpful fellow modelers on forums such as this one.
Be sure to state what aircraft, armor or whatever model you are working on and to specify which color scheme you want to do.
Regards,
Jimmy Cancino
Valleyfield, Quebec, Canada