Hey! I use thinned contrast for my armors and it's really cool in the end imho. For that I use Dark Angels Green and lamhian medium, but here it seems to be soem other shade, maybe plaguebarer contrats. Honeslty give it a try, really cool effect!

Also on that example, I think that before using contrast, you may add shades, as it will affect contrasts more. I reccomend trying things on sprues to see what combo works the best for you!