I have read about them and seen them used a few times before. I also heard of one person who had a filter made especially for her. She called it not a warm up, but a pink up. I think they warm an image and take the cool eldge of an image without giving rise to that jaundiced look that using a yellow to the point of noticeable warmth gives. The woman who use the pink up used it for scottish mountain scenes in snow. It looked great, reducing the blue cast and really adding warmth...but red rather than yellow warmth. This one was pretty subtle. I guess redhancers would be useful for autumn foliage etc.