They can be. NANO's are like a loud Spartan II, with the capability to use stealth. NANO's were designed as the ODST of Spartans. More durable, like shocktroopers. However their creatores prefer less blunt battle tactics...
But no that picture, the darkness and the redness was meant to represent the theme of the current NANO's, and more specfically the one in the picture. Darkness...