Mono = one, Chrome = Colour.

A black and white work fitting from the very dark grey (black) to the very clear grey (white), with the various shades of grey in between, is a monochrome work.

A black and white work toned entirely in a single colour will remain eligible for the monochrome class. For example: sepia toning, or if you turn the whole photo blue.

A B+W work modified by partial toning (or addition of one or more colours) = colour photo, and is not eligible for Mono. For example: in portraiture, if the whole photo is B+W, but you colour-pop someone's blue eyes, then it becomes a colour photo.