There are many ways to make monochrome images, which although we may usually mean black and white can also mean all sorts of toning techniques, such as Sepia, Gold, Selenium, Blue.....and so on. These all originate in principle with film and paper prints and the various colours are created by the type of photographic paper (bromide paper is black and white, chlorobromide paper is warmer black and white) and various chemical treatments. Often this calls for a bleaching process followed by a toning process. Sepia is gorgeous, Gold a bit OTT in my opinion but leading to super-archival prints. Selenium toning is highly toxic and should be done outdoors or in a very well ventilated area. Blue toning is a direct toning process, and rarely used as not many images suit it. We can also use food dyes, as conventional photographic paper has a gelatin base which takes up the dye.
So monochrome seems to be, potentially, quite colourful. But it is just one colour. Include two colours and it is no longer a monochrome print. With digital imaging, Photoshop and other programs follow the original concepts and effectively simulate the film/paper processes. The terminology is usually the same, with minor variations such as grain becoming noise.
I love monochrome photography and there is a sheer joy in seeing a truly well printed black and white image. We could think of Ansell Adams, but also Selgado and others, who produce startling good images. I am convinced that the best black and white is with images that just would not look right in colour. It's more than just converting in Photoshop. This is why when shooting black and white I set my camera to record black and white JPEG images. My classic set up is the Pentax Q-S1 with its standard prime lens.
If I do want to make a monochrome image from a colour shot, then I use Channel Mixer in Photoshop and adjust the colour levels to simulate the use of filters. So, here are my three competition image, two are in-camera BW JPEGs and the other a sepia conversion.
The Slaughterhouse, Dunham Massey
Dunham Cottages
Clara's, Whitby