Imagine, for just a moment, that you are a landed noble from a long and storied family that has served king and country for centuries. Now imagine you are out in your carriage, only to see it made to look rather shabby next to the absolutely sumptuous affair nearby- a silver, cream and carnation confection of a conveyance. And whom does it convey?
Miss Gertrude, a commoner in the printing business.
Sumptuary law is the English name for the category of law that restricts luxuries to certain classes of people. These laws exist for many reasons- in some cases, for example, they are meant to alter the flow of trade. However, they are more commonly associated with laws designed to make it visually obvious who has the higher rank, especially when the money of those with said rank is not up to the task. And then there is that other version- sumptuary laws based on religion.
If there are going to be out of game requirements for clothing, it would be worth considering having in-game sumptuary laws related to said requirements. This could be as simple as “The Lord of the Land has stated that all vegetation-based beings wear green,” or as complicated as any puff-up medieval dandy could wish.
On the other hand, I would be careful before altering your games established garb rules or creating in-game penalties for what might, honestly, be an out-of-game problem regarding cash. Costumes can be a rather expensive outlay relative to your players budgets, so make sure they are willing to dash out and buy or make more if they need to.
A third option is one that may appeal to players who primarily enjoy the game through dressing up; considering how inequitably sumptuary laws were enforced, it would be possible to generate a plot around the creation of these laws that only effects the players interested in it. After all, most non-martial nobles have very little interest in the dress of the man pointing a sword at their guts- but a great deal of interest in the doings of other non-combatants.
Image Credit: Burmese Upper Class Couple by Philip Adolphe Klier [Public domain], via Wikimedia Commons.