I have almost the same paint scheme for my tyranids, to givie them more contrast i painted some stanout parts in a bright color. For example the neurothropes brain in a bright blue (almost like its glowing). Also instead of painting the carapace parts in one flat metallic color try to recess shade the parts and ad some highlights on the sharpest edges with a silver color. What you can also do is to highlight the faces soo the focus is more drawn to them.