I don't think it's the colors. At least not with the Hudes. I think it has to do with the positioning of the head and eyes, which Le Fanu touched on:
I don't think it is a question of just the faces, but I do think that if faces look "out" of the image, away from the viewer, slightly bowed, it does give the image a more wistful, sadder feel.
I would amend this to suggest that, at least with the Hudes (I don't have the Aquarian at hand), it is when their eyes and/or faces are cast downward that the "shuttered" feeling of dampened, numb emotional states comes into play.
This doesn't necessarily mean sad. It simply means an absence of strong emotion--neither strong happiness or excitement as per, for example, the gushing Robin Wood Tarot, nor strong anger, pain or fear as expressed in many other decks. When their eyes and faces are tilted downward, they seem to be in their own introverted, touch-me-not, numb-to-the-world reverie. And this can be interpreted by many as melancholy, even by me. (I don't like The Lovers in the Hudes, for instance. Everyone is looking down).
But when their eyes and faces are turned straight-ahead (even though not straight at the viewer, but looking off at some other point we can't see), like the Hudes' Two of Swords, Queen of Swords, King of Swords, Two of Cups, Nine of Cups, etc, etc (there are quite a few), they appear much less shuttered and thus more neutral to the viewer. They are facing their world head on--neither smilingly nor sadly, but they are looking at it in the eyes, and this throws off the emotional "shutters" that come about when a figure's eyes (the "windows to our souls," in many human cultures) are hidden or turned away.
Amazing how attuned our species is to eyes. We become discomfited if we can't see another creature's eyes because we can't determine its emotional state, and we also become discomfited if a creature (insect) has "too many" eyes because then we have no emotional center to assign to it; we feel it has no such center.