The short answer is it’s all about light and hues or the lack thereof. Looking for some clarity? Well, I think I can help with that to some degree so lets dive in!
Monochrome photography employs light in differing amounts instead of color to capture or represent images. Monochrome photography takes only a single color and uses a range of tones of that single color. Monochrome photography is not exclusively black and white. Other colors and tones can be used so do not feel limited in just using black and white. So long as it is a single color is selected then it would be considered monochrome.
Selective color is an effect that allows you to fully or partially desaturate while preserving one or more focal colors. I like to use this technique with in all sorts of images, you can chose just about anything to be a subject really. Most image editing programs will allow you to automatically select a hue from the photograph and then apply the black and white treatment except for the selected hue of color. Personally, I tend to edit my selected color images myself by applying a black and white filter and then erasing the subject area that I want to focus on using my phone screen, a tablet would work just as well but I’m in the market for a new one.