Printing duotones in traditional, non-digital printing can save a client money. Moreover, digital printing can look really cool.
Start with a duplicate file, since
the conversion destroys the RGB or CMYK original. Right/Option Click the background layer > Select Duplicate Layer > SET TO NEW > Name as file_duotone.
Image > Mode > Grayscale > OK
Image > Mode > Duotone
Set type to Duotone
Click Ink 1 well. Set color by method of choice, such as a web hex code.
Click Ink 2 well. Set contrasting color by method choice.
Adjust curves to the left of the ink wells to modify contrast.
Image > Mode > RGB or CMYK, only if the image is not to be printed as a duotone.