Thanks Oshyan. I've worked on the lichen a bit more (though in this iteration, I may have masked out too much). I noticed that I did use the voronoi billows after all, but now applied a little warp to get the 'fringed' edges.
Color variety is enormous, but may also depend on the photography. I also attempted to get less saturated centres, with fringes that were freshly colored, succeeded a bit, but I didn't find it the case in many lichens, so I dropped it again.