All chickens reduce their egg production as the daylight hours drop below 14hrs. Most purebreeds like Welsummers stop completely during the darker months when daylight is below 11 hours or so. WS's are generally not good winter layers. Also at this time of year chx start to moult and the older the are the heavier and earlier they moult. Birds in moult don't lay.
If your WS are moulting or starting to go into moult don't expect eggs. Even when they get their new feathers they will probably not start again until the daylight hours start to increase in late Jan early Feb.
If you want winter eggs get hybrid hens and consider lighting their house so there is 14hrs of light. Usually the morning hours are extended more than the evening if you want to do this as you need to still lock them in at night. I wouldn't bother doing this with pure breeds particularly WS as they aren't prolific layers at the best of times even in purebreed terms. Even hybrids don't lay as well in winter after their first year.
I suspect there's nothing wrong with your WS just give them a rest and carry on feeding them as normal and don't try to restimulate them into laying. Laying performance in anycase isn't just breed dependant it's very strain dependant. My Welsummers are still laying as well as they have through summer (which wasn't particularly great) even though they are into their 4th year.
I've not heard that dark egg layers are poorer winter layers and I suspect that there's no genetic reason to link dark egg layers with poorer winter laying. Though I stand to be corrected. Certainly my Vorwerks (Ivory cream eggs) are poorer winter layers than my Sussex (tinted brown eggs) and I don't think this is true of hybrids. Black Rocks for example lay brown eggs and are renown winter layers. Maybe it's because dark eggs are more difficult to find in the winter gloom so it just appears they are
HF