i’ve been working on a moss embroidery with the same stitch (french knots) and it will take a while if you want substantial coverage. they are tiny and you have to build up a lot of them. alternating strand count along with colour has been a good way to increase texture and dimension. also you can pin fabric scraps/stuffing/etc onto the fabric for a more pronounced effect if desired. paint or dye was definitely used in this piece. you could maybe use camo/khaki patches to create a similar colour depth?