snay1345 is correct about the use of a red filter. Back in the days when we shot film, I would often have a yellow filter on my lens as it gave some detail to the sky, and contrast to my subject. I had a red filter, and if I was shooting a landscape with big fluffy clouds in the sky, the red filter would bring out the clouds, and turn the sky black. Now I shoot digital, and start with a raw file and get the best color image I can and only then move to Silver Efex Pro. On the right side of the screen you have 18 different film types that copy what you might get if you used that film. There are also color filters that you can use to see what that would do for your image. I really like the program and am looking at my site going almost all b&w. But it was a good question, because I don't think lots of the people today have ever shot with film, and don't understand how those filters worked or why or when to use one.