The AQMF rules have damage from Green Gas and Black Smoke take effect immediately, so there is no need for markers to denote their use. But if you want some theatricals to add to the visual impact here is a good way to do it. And it is pretty inexpensive.
Go to the craft store (again) and scope out "Pom-Poms" over in the sewing supplies. They come in several colors and sizes, but two are particularly interesting: green and black
Forgive the fuzzy image (no pun intended), but the autofocus would not lock in on the fuzzy pom-poms
Herewith, a tripod wading into a cloud of black smoke
And here one follows up on a Green Gas barrage:
You can stack four pom-poms to get a pyramidal shape cloud or use them side-by-side. If you cut through the diameter of the pom-pom, you find that it is a bundle of fibers held together by a small ring in the center. So, if you want the more free-form gas, just pull the fibers out of the ring. The small cloud is just one pom-pom pulled apart. Clearly this will need some kind of base, but that is not a problem.
If they ever decide to move to limited ammunition for Black Smoke and Green Gas, leaving these on the table could mark the expenditure. An enduring cloud of either one could also become an obstacle on the battlefield