I think National Geographic may have a good thing going with their guidelines. Perhaps a good rule of thumb to draw from that is "Does it look the way you saw it when you pressed the button?" If not, it's not representative of the original scene and therefore questionable for photojournalism purposes. Maybe good art, but bad photojournalism.
As for colorization, when an image is colorized, it ceases to be the original image and becomes more of an "artists rendering."
Which I don't see anything wrong with, by the way. I enjoy colorized images as a way to look into the past, but not as if I am looking at an "enhancement" of the original photo. It's a different animal.