I would recommend using a non-chlorine bleach based color remover similar to this: https://www.ritdye.com/products/color-remover-2/
There are similar products on the market that you can use to pull the color out of the fabric without excessive damage to it. Here are instructions on its use: https://www.ritdye.com/instructions/how-to-use-rit-color-remover/
I would suggest using a "color remover" like the product above which will lighten the dark brown and then thoroughly wash it, then use the dylon olive color again.
As for the stain spots that you currently have, the color remover will help with this, but not fully remove it. It will also depend on what the stain is from, oil, specific chemical?
I would definitely consider getting some green tones back into the uniform when you are able.
As Blackwatch noted, it will be a bit more of a challenge for dyes to absorb in the same manner with the introduction of various acidic washes such as vinegar and even this non-chlorine bleach color remover, but it will still allow you to get more green back into the uniform.