elmo_training_outfit
The outfit the Dolls wear in the Elmo (vehicle) during training in Girls' Frontline 2: Exilium. In pre-release material and some in-game CGs, the outfit was originally depicted as more color-coded (ex. post #4450707), before becoming more consistently grey by release with limited color-coded.
Alongside appearing in fanart for Dolls which have appeared in the game, it is popular to depict Dolls that have yet to appear from elsewhere in the series wearing it.