A dictionary definition: “a photograph in black and white or in varying tones of only one colour.”
The easiest way to explain this in more detail is with examples.
Monochrome is often treated as Black and White also known as Greyscale…
However as per the dictionary definition the image can be any single colour
Sepia is a common choice for monochrome…
However any single colour is acceptable…
Blue…
Red…
NOT ALLOWED: Note that colour popped images are not acceptable as they use more than one colour. For example the following image is NOT acceptable and would be disqualified from a monochrome competition…
