Gritty and realistic is very hard to do I think, I'm still trying to figure it out myself. It's not easy to make black look good either - not as the main colour anyway. It's a nice idea you have for the colour scheme, but miniatures are so small that you need to paint the details to make them stand out more, for marines most people like edge highlighting to make the armour "pop" (grey or metal highlights would work for black).
It could just be the photo, but the ork skin does look a little bright. Other than that I think he looks very good. Those checkered dags must have been very hard to paint, I wouldn't even have the courage to try!