Select a colour photo with plenty of varying brightness’s and open in Photoshop.
Open the channels palette (window / channels). This has four channels. The top one is the RGB channel, which is the colour combination of the three grey scales of the red, green and blue channels. With all four eyes of the channels open the photo is displayed in colour.
Now click on the top RGB eye and drag it down to the next channel RED. The photo will now display the grey scale of the RED channel.
Go to the history palette and click on the camera at the bottom. This will make a snapshot 1 of the photo in its present state.
Go back to the channel palette and click on the eye of the RED channel and drag it down to the next channel (green) and in the history palette make a snapshot 2.
Drag the eye of the GREEN Channel down to the BLUE channel and make a snapshot 3 in the History palette.
Double click on the name ‘snapshot 1’ and type in RED, now double click the name ‘snapshot 2’ and type in GREEN, double click the name ‘snapshot 3’ and type in BLUE.
In the history palette you now have four snapshots. The top one is the RGB colour. Click on it and you have the photo displayed in the combination of three grey scales of Red, Green and Blue. Now click on the next snapshot Red and the photo is displayed as a red grey scale. By clicking on the red then the green and then the blue snapshots you can observe the changes in the photo.
Decide which one you like best, or maybe you like two of the channels, say red and green. If this is the case then we can mix any of the channels to create other versions of the black and white photo.
Go to the layers palette (windows/layers) and click on the circle half black, half white at the bottom of the palette. From the pop up menu select channel mixer.
Tick the monochrome box at the bottom of the palette and adjust the sliders to 50% Red, 50% Green, and 0% Blue and click ok.
In the history palette make another snapshot. This will be the combination of the Red and Green channels. Double click the name and type in ‘Red and Green’.
By adjusting the three sliders in the channel mixer you can change the appearance of the black and white photo to your liking, and by making a snapshot each time you make a change you can compare the differences. Click ok.
In the layers palette double click the channel mixer layer to reopen the channel mixer, and then you can readjust the sliders to a new value. The total value should add up to 100%. Flatten and save as and enter a new name for your Black and White photo.