Converting Photos to Monochrome but One Colour with Darktable

One of the most exciting things I think modern photography has is the ability to make special effects thanks to the computer. I have always been fascinated by mixing colour with blank & white elements. So, after giving a fast reading to the Darktable 3.2 manual, I found a way to create the effect I want: blank and white but one colour.

Who doesn't remember that dark scene of the Schindler's List film where a girl in a red jacket is walking. Then a few minutes later, you identify her corpse from a pile of bodies. Well, I will show how to make this happen, but with a more joyful image.

The picture above was taken by me with my Canon 90D in the last Christmas Parade 2020 in my local city. The left picture is the original JPEG from the camera, the middle one is the product of the RAW processed with Darktable and the right one is the same photo with the additional effect of a monochrome photo while keeping red-orange colours. There it is what I did.

The Colour Zone Module

I think this is the more artistic and right way to do it. The thing here is that while you see an orange train, it is not a plain orange, therefore I think because this tool allows you to run from full saturation to monochrome gradually through the colour gamma, it is the best to use.

By selecting the saturation tab, just keep high the red/orange area while dropping down everything else.

That's it! But, what if the photo had some other red element that I just don't want to keep the colour. Easy, do not forget about masks. For example, If I wouldn't want a colourful Canadian flag, I would have to create a mask around the train and apply an inverse effect. That would do the magic.

Keep the world colourful with Opensource! Good luck!