In this theory a underskirt is worn, either between the kirtle and gown or under the kirtle. In the former just the gown needs to be lifted to show the differing fabric at the hem. In the latter both gown and kirtle are lifted to show the underskirt.
1495-1500, UNKNOWN MASTER, Flemish, Lamentation, Museum of Art, Santa Barbara. The women in the back, right is wearing a dress kirtled up to show a rich brocade underskirt.
1470s , UNKNOWN MASTER, Flemish, Deposition, Wallraf-Richartz-Museum, Cologne (detail) In the bottom left you see a lady wearing a side laced kirtle. The red showing at the hem could be an underskirt. When both gown and kirtle are lifted the smooth front of the kirtle would show at the neckline while the red underskirt would snow at the hem.