A comparatively simple special effect to accomplish during production of The Original Series was that of phaser fire; which relied upon animation. As explained by Stephen Whitfield in “The Making Of Star Trek” (Ballantine Books, 1968): “A series of drawings is prepared, each showing the beam slightly larger and longer than the previous one. These are shot, one frame at a time, and when the film is run at normal speed, it appears as though the beam were being fired.”
In the image below, a piece of raw film is marked up to indicate the direction of travel for the desired phaser beam to be added to the scene. Apparently, an initial path slightly to the right was crossed out in favor of one to the left…
A screenshot showing the finished version of the special effect is seen below …
A great variety of beam colors were drawn by the graphic artists; as is documented in the following group of episode screenshots. Specifically, colors ranging from red and pink and orange to blue and violet and white are seen. Presumably, the different colors represent different force settings such as stun, heavy stun, disintegrate and kill, etc...
