Orange is invariable when used as a color adjective, meaning that it won't change to agree in gender or number with the noun.
Une chausette orange, Des chausettes orange, Un chapeau orange, Des chapeaux orange
Check out the Tips and Notes under Colors where they explain.
Information can also be found at: https://www.thoughtco.com/invariable-french-adjectives-1368796
The difference is that in your example, "Les jupes sont blanches," the color is a predicate adjective, which is linked to the subject by a verb, (sont, in this case), and must agree in number and gender like any other adjective.
In this sentence, "J'ai des chaussettes orange," orange is acting as an attributive adjective, modifying the noun directly, and follows the special rules for colors as described by ongaku81.
I think generally you need to be more flexible with certain things as direct translations. The point is to communicate, not translate, right? I got an earlier one wrong because I didn't translate "Le vetement" as clothing ITEM, but then as soon as it was "Les vetements" it became just clothing. I've rarely if ever heard anyone say "clothing item." And sorry, it shouldn't matter whether I am in the UK, US, or Australia--they should all be acceptable. Loosen up on the word for word translations.
There are many things like that in French. There are many more ambiguities in spoken French than English unless additional information is added over what would need to be added in English. The French frequently adapt other methods of communicating "missing" info when it is needed. Example: "C'est sa voiture," If it is actually important for the recipient of the communication to know the gender of the owner of the car (maybe talking about two people with different genders who own cars) they will increase precision by saying “C’est sa voiture à elle,” or “...sa voiture à lui.” Another example: If it becomes negative then what ongaku81 is saying about "des" being the indication of plural is no longer accurate and you wouldn't know unless otherwise indicated in spoken French: "Il a des chaussures," BUT "Il n'a pas de chaussures." Unless further specified the negative could be either plural or singular (feel free to correct me or chime in on this onga). I suppose listener and speaker relies on context? Again, feel free to chime in Onga.