“The Klingons from the original series were born out of the concept that the way to create an evil adversary was to paint his face dark brown and put a black wig on him and make him nasty and this is a Klingon. Then, as Klingons moved into the feature films, the makeup artists put a little ridge on the forehead. When the new makeup team for TNG was brought on, they took it a step further. Using Worf as their initial creation, they not only kept the ridges on the forehead, but enhanced them. They went heavier with the ridges so they would stand out in much greater relief. The bone structure was so pronounced and close to the surface of the skin that the Klingons evolved to a near-exoskeletal creature. It was clear that the Klingons were not just versions of human, but definitively of another race.
The design premise of the new Klingon foreheads in TNG was based on the look of prehistoric dinosaur vertebrae. By using dinosaur anatomy books, the makeup design could be created from just a small portion of a single vertebra. That accounts for the enormous variety of Klingon foreheads.
In addition, the makeup designers, wanting to accentuate more of the menacing otherworldliness of Klingons, modified other facial features. They added a ridge to the nose that interlocks with the accentuated forehead bone structure, making it more fearsome. Changing the teeth became important, because now that the foreheads were enhanced and the nose structure was enlarged, it was suddenly evident that the Klingons had Hollywood white, perfectly straight teeth. The contrast between the upper skull and the face and mouth was suddenly overwhelming. Therefore, the makeup department started experimenting with creating acrylic yellow, snarly, craggy oversized teeth that were applied as caps directly over the actor’s teeth. Michael Dorn was the first to wear the appliance. Thereafter, every speaking Klingon has worn upper acrylic teeth, saying his or her lines through a special set of caps fabricated for each actor and given to the actor two days in advance of shooting so he or she could get used to speaking through the appliance.”
Source: Star Trek Aliens & Artifacts [Michael Westmore et al.] 2000