I dunno, coats? Nah....that won't work. There are these things called "feathers"...oh wait...they already have those!
And all smart alecky comments aside...that is all they need! Especially in your relatively mild climate. Up here in the great white north and I still do not provide any extra heat. I brought in a building this summer that a guy was unloading and he had a brooder light in the rafters to use as a heater. I chuckled about it and he did not take it well. SOME breeds, notably the single combed varieties, are prone to frostbitten combs and in severe cold some chickens will get frostbite in their feet and legs. I can remember an issue of Poultry Press over ten years back where the editor was culling a large number of bantams after a cold snap due to frozen feet. Our largest number in terms of breed are Brahma large fowl. I have kept some groups over winter in a south facing run in (no south wall) and they have piled in the corners on the hay and done well all winter.
Your pics show good sized, healthy chickens who will ride out winter just fine. Provide wide perches so they can roost on their legs and feet. Get the food and water off the litter as it is a source of disease and parasites. Your ventilation looks good but I would put a couple of small vents low down to provide the convection necessary to dry out the litter. Make sure your Silkie has a way to get out from under the other birds and stay dry. Paint is your choice but it looks nicer and makes the wood last longer and this is good PR for you, your birds, and chicken raising in general.