I have not heard of a night guard. But I'd imagine it is the same thing as any guard dog. I prefer a dog 80lbs to about 135lbs. If you get much bigger than that they often lose some of their agility. But I understand some smaller dogs also make great guard dogs. If we look at the Belgium Malinaois they are on the small side and yet they can still take a man off of their feet. they range from 55lbs to 75lbs. They excel in Schutzhund. But for my taste I prefer a larger dog. If you are looking at color black is considered the best color by many. Some like brindle. I asked why black. I was told that people are more intimidated by black dogs. My girl Grace and her pup Princess is a black brindle. You can see them at night. I had the light on on my deck last night when I let them out. I went to check on Grace and couldn't see her until she started moving. After relieving herself she began running like she was hunting a rabbit. She looked like a shadow.