Well, that was a part of the plan; the MC will have a something like a modiste who will give them a crash course about fashion, but more often than not the servants will advise the MC what would suit them/what they should wear, versus what they should avoid being seen in (although in the end, it's the MC who has the last say, of course).
Truth to be told, all in all, it's not difficult, nor a lot; just plenty of *if(s) (and knowledge in the subject). It would look like something like this (which didn't take much work, I wrote it just now):
*if skin_tone = "tawny"
You should avoid shades that are 'in the middle'; you either dress dark, or bright, but never neutral. With that hair color
*if hair_color = "blond"
you better avoid yellows and gold, too.
*if hair_color = "light brown"
you could try... no, don't ever think of using black.
*if hair_color = "dark brown"
don't ever think of brown or mauve; I'm being serious.
*if hair_color = "auburn"
you should steer clear from red, and be really careful of olive colors.
*if hair_color = "black"
wearing cool tones such as mint is a bad idea.
Although you have magnificent eyes; it would be such as waste not to make the most of them.
*if eye_color "blue"
Try to wear shappires or chalcedonies, maybe pearls.
*if eye_color "green"
*if hair_color "auburn"
I have warned you about olive tones, but a set of apple green emeralds will do the trick.
*if not (hair_color "auburn")
Emeralds of the same shade will do you good.
*if eye_color "brown"
Cat eyes or amber will be wonderful on you.
Mmmm... I can do that. I can give the MC a few options regarding their features (but they wouldn't be really specific), like:
#I take after my father; I have a long face shape and strong cheekbones.
#I take after my mother; I have a button nose and square jaw, with a small forehead.
#I don't look like my parents; my features are...
[Then you would have three or four more options.]
Though these would be a bit more hard to introduce in the narrative, they could be used, too. A MC who takes after Datura would get (even) more hate from Aquilegia than someone who might look like her, for example.
P.S: ugh, I should be working on the CSComp