Light filters can be put over the top of the lens in order to change the coloring of the light beam. Changing the color of the light offers a different function depending on the color. For example, red is very useful for night vision, which would often be adopted by the military and SWAT teams advancing upon an enemy. Hunters tracking an animal over long distances through the night would also use night vision. Blue and yellow light filters are used a lot for tracking blood.
These colors will illuminate the red of blood in dark conditions, making it easier for hunters to track a wounded animal. Police may also adopt this method for tracking a wounded criminal who has been shot but escaped from a shoot out. Green is a very useful light filter that is often used by hunters.
Green will not frighten animals and so you can preserve your location and vantage point while maintaining a track on them. Green cannot be seen from peripheral vision either, which makes it useful for military purposes as well. Typically, only the best police flashlights will have these filters.