It's called "$mirror: (0/1)" and has now been set to 1 for the transports.
And are we sure that there is in fact a Colossus icon?
$start_icon
$type: 6
$team: Friendly
$class: GTVA Colossus
$pos: -341.618774, 441.265167, 5225.889648
$label: Colossus
+id: 8
$hlight: 0
$mirror: 0
$multi_text
$end_multi_text
$end_icon
That is what is in mission file, but it is indeed clearly showing a Hecate in there. (this is from stage 2, same stage as the screen shot)
EDIT:
Okay, it is supposed to use "IconT-Super" but for whatever reason is using "IconT-cap"
Edit Edit:
"$type: 6" is supposed to be "$type: 31". Problem solved.