Ive got 2 ideas of a version of this (both versions alien vision can see through the fog just fine:
1: the collosus can excrete fog as a huge cloud around itself in order to hide and make itself harder to hit (when the eye glows humans can see it but other than that it’s hidden)
2: the collosus can excrete a large area of smoke beneath itself which completely hides other aliens which are moving under it, this fog not only visually hides aliens but also hides them on the map by temporarily removing los in that area.
This would mean that when humans finally kill collosus they would suddenly discover a swarm or goliaths or all kinds of aliens underneath it which have crept up to their front lines in the fog.
Could also make it so the fog doesn’t work when it’s near certain human buildings so it’s not too op
Another part of all this would be that human ai would be unable to target enemies within the fog, so that could potentially be a bit op too