This is an exercise to help understand colour and how in black and white imagery it gives powerful tonal control allowing emphasis on some objects in a scene while suppressing others.
First of all I took a colour photo of a still life scene containing the four colours red, blue, yellow and green.
Then, using Photoshop, I converted the colour image to Black & White. Following this I converted the image another four times, each time simulating the effect of a different coloured filter – red, blue, yellow and green.
On the right hand side is the same photo, converted to Black and White.
As can be seen the blue background has been rendered almost white, whereas the three other colours have been darkened considerably.
The blue of the background has been turned much darker, but the green and yellow turned lighter. The red of the tomato in the centre has turned much lighter.
The image on the left has used a yellow filter. The effect is similar to the red, but not as strong – the blue background is still dark, the red tomato is lighter than the original B&W photo but not as light as when the red filter was used. The greens of the limes are a little lighter.
The final photo shows the effect of a green filter. The blue background slightly darker than the original B&W photo, but not as dark as those produced by the yellow or red filters. The green of the limes is lighter in tone than the original and all the other filters. The yellow of the banana is lighter than both the original and the blue filter, but a darker tone than that produced by the red and yellow filters.
What have I learned from this exercise? I had, many years ago, used red and yellow filters in black and white film photography, but had forgotten most of what I had learned. This exercise brought those lessons back!