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. 

A B+W work modified by partial toning (or addition of one or more colours) = colour photo, and is not eligible for Mono.