The phaser used in Star Trek: The Motion Picture (TMP) and Star Trek: The Wrath of Khan (WOK) is pretty much the same.
The pistol phaser from the TV series was updated by prop master Richard M. "Dick" Rubin for Star Trek: The Motion Picture. His design was patented by Paramount Pictures. (Rubin was also the propman for the Outer Limits, which aired from 1963 to 1965.)
It's really really hard to find a photo of the phaser used in TMP, but I have found a copy of the patent application and a superb replica.
The TMP and WOK phaser had the same basic design: A rounded, flattened obloid on which the backward-raked grip was set toward the bottom-front. A groove in the front of the body (which wrapped around the front and the slightly the sides) had an acrylic "crystal" set into it, which functioned as the phaser's "emitter". The groove extended around the sides and the back. About halfway back, a groove ran up the left side, across the top, and down the right side -- as if this forward section was the power pack, and could be removed to make a "jewel phaser" (as in The Original Series).
At the back of the main body was a raised translucent white dome inset into the phaser. This was the dilithium crystal cell, and was also made of acrylic.
The trigger in the grip was cut out of the grip itself, and protruded slightly. The bottom of the grip was black. Where the web of the hand met the grip, a slight indentation existed.
The forward "power pack" section of the TMP phaser contained the control panel. On the left were four colored panels (front to back: dark green, red, yellow, blue) with two tiny white lights into the lower corners. In the center top of the panel were two rectangular, copper strips side by side. Below them was a glossy black square rimmed in white. On the right side of the panel, at the top, were two more colored panels with tiny white lights (yellow top, light green bottom). Toward the back was a rectangular silver panel, only slightly longer than the black panel it was next to, with tiny white lights in the upper right and lower left corners.
The TMP phaser was painted a very specific color: Krylon Stone Gray #1306. This is the phaser to the right in the picture below.
In the prototype, the lights on the left-hand side of the panel were lit and blinked on and off. Astonishingly, this phaser cost a hefty $4,000 each to manufacture. To save money, the lighting was eliminated. This meant that the phaser got smaller by one-third! This was the production model used for the film (15 of them were made).
Here is a somewhat inaccurate replica of the WOK phaser.
Initially, prop master Joe Longo designed a new phaser (and communicator), bu Paramount executives rejected them in favor of a slightly modified phaser from TMP.
The WOK phaser replaced the cut-out trigger with a push-button one. Ribbing around the groove on the sides and rear of the phaser body were sanded off. Another change, barely noticeable, is that a metallic reflective rainbow graphic now ran along the sides of the phaser in the groove.
The color was chagned to Plastikote light metallic blue #1348, and the color of the bottom of the grip changed to match the body.
Aside from the color change, the biggest alteration on this phaser is the control panel. All the colored panels are gone. They are replaced by copper-colored panels instead. The arrangement of the buttons on the panel remains the same. Each small button now features a little double-stripe of white along the bottom -- as if it were a sliding toggle. The four panels on the left side of the phaser now feature a single light at the inner top of the panel. They were designed to light up, and this was accepted.
Here's the WOK phaser being used by the doomed Capt. Terrell.