Technically the color of any of the wepons would be "Tuneable" since the radiated color that is visible would be waste energy and not related to the weapon. At least when you consider the fictional logic on how these things work, lasers on the other hand would be various colors depending on the frequency of the emission red for low power bluer for higher power and would still be due to wasted "lost" energy from the beams.
Phaser and Disruptor colors are by cannon due to impurities in the crystals used to collimate the beams, the color of the field surrounding the matter/anti-matter in photon torpedos is a function of wasted radiated energy, Plasma Torps would be the only consistantly colored weapon based on cannon information I have read over the last 3 or 4 decades.
According to the TNG Technical Manual, the Photon Torpedo is your typical guided missile-style weapon just like we've ssen them in Star Trek 2, 6 etc.
The glowing effect around the torpedo that appears after launch is some kind of casing that if put around the actual torpedo.
My guess is that they are kept there with some kind of forcefield and that the entire idea is to protect the torpedo from interstellar gases etc.
However the science in this is somewhat bumpy, especially when we consider that the torpedo has its own navigational deflector.
So technically it does not really matter what colour the glow has since it's the actual warhead that counts.
Just like with the beams it's dependant in what tech is used to produce that beam.
In the old SFB materials it is even stated that a "phaser" is some kind of directed energy weapon with capabilities comparable to its Starfleet counterpart.
In case of the alien Andromedans it is even the case that their Phaser 1 and Phaser 2 work on entire different physical principles.
As for the Plasma Torpedoes thers is an interesting aspect that is usually overlooked.
In the Original Series the big red ball fired from the Romulan ship is generated by the ship itself.
It is a large amount of plasma encased in a forcefield.
Just like it is represented in Star Fleet Battels and Starfleet Command.
During the Next Generation era, the familiar green glowing Plasma torpedoes are actually solid-cased torpedoes, just like Photons (they were actually abbreviated this way in TNG sometimes, like in Q-Who)
In DS9 Season 7 episode 1 "Image in the Sand" Odo tells Kira that the Romulans are stockpiling Plasma Torpedoes in the Hospital on the Bajoran moon.
I mean how can you stockpile balls of plasma in forcefields?
well, as far as plasma torps go, I wonder if it would be possible to have 2 settings for HYT for the plasma, an option to fire a salvo of torps like with quantums, and photons, and the other for the slow moving plasma wave, I doubt it star trek romulans or anyone else using plasma torps would be limited to either firing plasma torps 1 at a time or as the current massive one at HYT, just a thought to add a little more option to the game. If you wanted it could be 2 different plasma torp launchers, 1 class shoots the reg salvo like photons and quantums, and the other deploys the slow moving "cloud".
4 minutes, 28 seconds into the scene "Bones, where's my torpedo?"
You can see Spock and Bones working on the actual torpedoe's guidance,
it only starts to glow as it leaves the tube.
And I totally agree it does not really matter what colour it has.
Besides, it changes arbitrarily between show and episodes sometimes.
For example the Cardassians had purple beams and torpedoes in their first
appearances and changed later on DS9 and the 7th season of TNG to yellow.
And I can tell you purple beams on Cardassian ships don't look really great.
A photon torpedo (I thought) is constructed of the elementary element of light. If so it can be any color based off of its vibratory frequency.
A photon torpedo is a big metal box with a matter/anti-matter warhead in it. The "Photon" part of the name probably comes from the fact that they are warp capable devices and therefor can move at (or above) the speed of a photon.
The colors are probably dependent on propulsion systems and launcher mechanics. Which would easily explain the different colors seen in the show.
Torpedoes use warp sustainers for propulsion/maneuvering and for warp travel when needed. Its stated that they release plasma exhaust at launch. Warp coils utilize warp plasma to construct the warp field. The tube has warp coils to hand off to the warp sustainer.
It might be possible that torpedoes house warp plasma which generaste thier warp filed through iteraction with the warp sustainer engine previously energized or inititated by the handoff coils in the tube.
If this is the case, the torpedo may be glowing like, and for the same reason as, the warp nacelles on the ships themselves.
Now this might imply that torpedoes should exhibit the same "glow" as the firing ship's warp nacelles...unless they utilize a different type of plasma in the torps. One might find it worthwhile to inject the plasma into the torpedo from the EPS just prior to launch, than to store it...though we know they can and do store plasma in Trek.
The color can also be an indication of the energy level of the warp plasma in the torp(not necessairly the explosive yield). It might not be a coincedence that Q-torps fly faster(is that canon?) if it turns out the warp plasms more highly energized, therefor possibly providing a stronger propulsion through the warp sustainer engine.
For instance, did the torp change color toward the higher energy spectrum when they increase the yield of a torp in canon?
Well if it is of interest (but unrelated to the OP topic)
In Star Fleet Battles (game) the Klingons had a ship called a Mauler. Essentially a D6 / D7 (light cruiser) built around a large weapon. Took time to build up a shot. Had other lighter weapons for defense. No I'm not a geek (or at least I keep telling myself this).