when you dye something that already has color, it's called over-dyeing.
As they are somewhat faded already, you could give them a long soak in something notorious for removing color, like OxyClean or comparable products, just to lighten them a bit more. Bleach proper is really harsh on fabrics and can eat right through natural fibers. If you were to bleach those to white, the fabric would be so weakened it might tear when you move.
As long as it's a darker olive green you are aiming for you will probably be okay. I think the little bit of blue left in those shorts will probably make them look really 'well worn' once dyed green, which may or may not work towards your end goal.