This photo I was photograph few years ago for my friends´ DJ gig. To take this photo is pretty easy. You just need to turn your Shutter speed really slow = so more light could get in. The focal F would be over F.10 or something. Turn on the flash. Then quickly focus photo on the light source of the dance floor, then move the photo to the persons you want to take. Release the shutter. Then move the camera around, so different light color could get in (since the shutter speed is really slow now). It seems a bit complicated, but since you try it, it would be really easy. Good Luck ! You could view more photos of this technique in my other photography album.