I don't think it's a colorant, as chlorine does have that yellow green color as a gas. Might be added to the liquid phase, but I'm not aware of a dye that would be stable in liquid chlorine.
Also, apparently you can improvise protection by peeing on a towel or other piece of cloth and wrap it around your head. That was the solution the Canadians had at the Battle of Ypres. Turns the chlorine into trichloramine, which, while unpleasant, will not form hydrochloric acid in your lungs.
That's just the color it is. I've worked with it. As a chemist and a stupid person, I decided to have a little sniff of it to see how pungent it is. It smells like bleach and pain.
I couldn't find anything from OSHA or websites of other safety-related organizations specifically about attics. The inclusion of this video of "never in an attic!" also seemed strange.
A lot of the procedures I did find about "shelter-in-place" (i.e. hide anywhere omg ur gonna die) involve "sealing the room", so I am guessing that attics are just generally not well sealed compared to actual habitable rooms in a building. I.e. air/chlorine from outsite will diffuse in much faster into an attic than a room right below it. That said, it probably depends on the elevation & construction.
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