You are not the first to think about making robe variations tman, and you won't be the last. The last word I got on the topic was the desired way to do this was with post processing. The same way that hair colour starts with one base image and 'shaders' change the colour based on character settings. My understanding is this would be less resources intensive than having potentially 100s of more individual textures/meshes loaded by the game each time you log in.
I see what you're saying about the efficiency, but these 6 textures combined, when zipped, only take up 976 KB. So, if colors were added for all currently available robes, it would add roughly 7 MB to the download, which is already around 1 GB. That's an increase of less than 1 percent, and it would more than double the body customizations available to the player (right now we only have normal, LA, MA, HA, and one robe).
And of course, if there ever comes a time when PS can change the colors of clothing arbitrarily (which sounds like a long way off), they can always be deleted.