Take or choose an image which contains at least two strong contrasting colours. Using the channel sliders or controls available in your software, create two opposite versions of the image in black and white. In one lighten the grayscale tone of one of the colours and darken the tone of contrasting colour as much as possible.
In the second version, perform the reverse. Ensure that before this process is begun, a default black and white version is made as a reference.
Original image with contrasting blue and orange
Default image with straight conversion to black and white with no adjustments
For the first image, I opened the black and white image and used the channel slider to increase the blue levels to maximum and the red levels as low as they could go.
I then reversed this do that the red was the dominant colour and blue was much lower.
I then decided to make an adjustment of the tones that I felt was a more accurate blend of the colour channels.