These Plague Marines have a techno-virus/cyberpunk vibe.
  - Prime with   Necrotic Flesh  because you didn't think ahead.
 - Paint all the armor areas with   Deepkin Flesh  
 - Paint the horns with   Abaddon Black  
 - Paint the Tentacles and tongues with   Calgar Blue 
 - Paint Trim and emblems with   Gryph-hound Orange 
 - Think about what to do next...