How do I take dark army green and lighten it up. Do I use white or yellow paint?
This depends on whether you wish to make the new color a paler shade of the same green or a different shade of green.
First I will suggest rather than adding the lighter to the darker, you will wish to add the darker to the lighter. Making a light color darker requires less paint than the reverse.
Now I have given you the advice you asked, I will give you some un-asked for advice. Paint is relatively cheap. BUY WHAT YOU NEED! If you mix colors yourself, you will never be able to replicate WHEN you will inevitably need to.