I do lots of night shooting in existing light. When you start shooting, take a couple fo minutes to spot meter the hot-spots, the highlights on people. Average that, go down a little and set for manual and go for it. For example, you might find that the highlights are 1/125 @ f4. Set the camera for 1/90 or 1/60 and go for it. Check it every once in a while or as you move around to see if the light is changing on you.
You might consider desaturating the images to B&W when you get back to the computer. That gives a more documentary feel and then you don't have to deal with the colors from the various light sources.
Something shot at night using the existing street and building lights. The color shift is from the street light.