So, in the OW entries for Slate, Fix, and the other Summer Knight, the catch for their Toughness power is "Trappings of Winter(for the Summer Knights)/Summer(for the Winter Knight)". My opinion, based on in-book anecdotes and logic, is that this is backwards. Summer Knights should have the catch "Trappings of Summer" and Winter Knights should have "Trappings of Winter". Now, I now this might initially sound crazy at first, so hang on to your britches as I explain myself.
Exhibit A: In Small Favor, Fix confronts Harry and Thomas while they are parked. There is snow outside, and it can be assumed that is rather cold weather. Fix is wearing unseasonably light clothing, and is noted to not be noticeably concerned about this. (If I remember the scene correctly.)
Exhibit B: In Proven Guilty, Lily provides protection for Harry's group from the environment of Mab's stronghold(Arctis Tor?), i.e. the extreme cold.
Exhibit C: In Changes,
Harry acknowledges that despite being Winter Knight, he has no protection from the cold.
Now, what does this show us? That the Summer Knight has no trouble enduring the winter cold, that the Winter Knight is not protected from the cold, and that Summer magic can protect other people from the cold.
This makes sense. You cannot easily fight fire with fire, nor cold with cold. While it should be a simple matter for Fix to use his Summer Magic to keep himself warm, how can he use magic related to fire, heat, and growth to easily keep himself cool? If the Winter Knight went up against the Summer Knight and attacked him with Winter's Magic, the Summer Knight should be able to counter the cold, ice, and decay with his own heat, fire, and growth.
On the other hand, knowing that the Knights' power is granted by another higher power(the Queens), what possible reason would they have to give the Knights resistance to their own power(assuming they even could)? There is no reason. On the other hand, providing protection from the opposing court's themes makes a lot of sense and would serve to make the Knights more effective.
In conclusion, the Knights of the Faerie Courts should have the trappings of their own Court for catches, as they are explicitly and implicitly not resistant to them.