Why Should I Use Colour Filters on Black and White Images?
Using a colour filter on a black and white photograph changes the contrast, and the shades in the image. A colour filter can be added post-shooting in an editing software such as photoshop. However, the editing software needs to be able to see the colours of the image to make any changes (you can't put a colour filter on an image that is already black and white). This is why it can be useful to shoot raw in black and white, so that you still have the original colour image to make changes to.
If shooting black and white on a film camera, using traditional colour filters might be the way to go, but make sure you think about how that will affect your shot as this is entirely dependant on the colours of your photo!
So What does a colour filter actually do? How does it work?
A traditional colour filter allows light in the colour of the colour filter you are using fully in through the lens, making that colour appear whiter in the image, and the filter’s opposite or complementary colour will appear more black. So for example, a red filter will make anything red in the image appear closer to white, and anything on the blue-green end of the spectrum will appear black. The red filter is fully letting red light in, and blocking blue/green light.
All of the colours in between the filter colour and it's composite will take on various shades of grey from white-black depending on how close in the wheel a certain colour is to the selected filter. Below is an image of how each colour appears when a certain colour filter is applied.
The yellow and red colour filters are quite common. Red will make blue skies appear more dramatic as the blue will come out darker, and tends to create a higher contrast, more dramatic look. Yellow and orange filters are often used in portrait photography to brighten the warm hues in a subject’s face. Blue is not commonly used as a colour filter for black and white photography as it can make an image look dark and low contrast. However, all this comes down to is the colour wheel, and how each colour filter will affect the colour’s particular to your image.
Below are some examples of images with different colour filters.
Now that you understand the basics of what colour filters actually do to a black and white image, you can start playing around with colour filters and see how they change your black and white images!
-- article by: Cynthia Banks.