This is the most complicated method but gives the most control.
Open your image & convert to B&W. If you've used adjustment layers to convert to B&W you need to flatten your image now to preserve your conversion.
Next go to > Image > Mode > Grayscale. Next go to >Image > Mode > Duotones and a dialogue box will open
Duo.jpg
A monotone is what is says a mono conversion but you can change if from Black & White to Red & White or Green & White or any other colour you fancy, & white.
What is more interesting are the Duotone, Tritone & even Quadtone options, or the selection of Presets
Duo 2.jpg
I selected Tritone & changed the second & third inks to a green & a mauve by clicking on the white patch & selecting a new colour.
To the left of each the ink colours is a box with a diagonal line through it - this is a curves box & by default each is a straight line at 45 deg which doesn't allow the different colours to tone different parts of the image
Duo 3.jpg
Continued in next post...