boiling your white t-shirt together with a lot of green walnut leves will colour it, the colour is an army like greenish brown, it looks great but if you're not experienced with dying fabrics it will come out spotted. i herd that there are other plants that give other colours but i don't know much about them.
If you want to do it the easy way you can allways tie die it, it's simple and looks good, plus there are lots and lots of ways to tie your shirt for dying so you have where to choose from
i guess this is not useful if you're an anti hippie anarchist or if you fucking hate the army, but it will help if you need to blend in with the bushes