What is a Transmission Color pass? I was trying to look up what a Transmission Color pass was in the Blender docs, and I only got this:


  Same as above, but for transmission BSDFs.


This was pretty vague, and I didn't understand what transmission was, so I tried a test scene. With the default scene in Cycles Render, I added a plane scaled by 5 at (0,0,-1). I moved the default point light at (-0.7, -5, 6). I changed the default cube's material to a Glass BSDF (I guessed transmission had to do with an object that's translucent), and I selected the "Color" Option in Properties -> Render Layers -> Passes -> Transmission to use the Transmission Color pass in the Compositing Tab. I rendered the scene, hooked up the "Transmission Color" output to the Compositing node, and got this. 


I still didn't understand what was going on, so I tried to google "transmission color pass", and I found a site that said this:


  Transmission takes place when light passes through an object without being essentially changed


I also got this from 1:02 in this YouTube video about what happens to light in transmission:


  It can be granted safe passage through the medium itself. Sometimes it'll be refracted...


So from all of this, I'm guessing a Transmission Color pass is a pass where white parts show areas that allow light to pass through freely and black parts show areas where no light passes through. I'm not entirely sure on this, so I was wondering if someone has a different answer as to what a Transmission Color pass is. Also, does this pass only output a black/white image, no gray/color?
Answer is in your first two lines. Transmission pass is nothing else than colors of transmissions rays. Imagine diffuse color pass. It's color only. No other influence like shadows and glossy. Transmission pass is exactly the same, but color of transmission rays. Make your cube red (in glass shader) and you get red color in transmission pass. 

PS: Does not make sense write the same few times in documentation

PS2: maybe you try find something that not exist :-)