The thing I find most useful when describing black and white images, especially when comparing them to colour, is the importance and relative ease with which they convey clean, clear messages.
The message may be obscure by nature, but the intent, the content, by it's very nature is simple, graphic and free of clutter. Honest.
I guess another way of looking at the difference is, mono images have one "emotion". That is to say, they they only offer one emotional opinion. It is somewhere between cold reality and stark certainty. The offered image has only one level of context and that is content.
If a little colour is added, through toning or if the images is in fact colour, just devoid of most colours, then there is, no matter how subtle (or even because of?), a shift in our pre conceptions. As humans we look for clues. Colour is a clue, tones are a clue and so is texture. Take one away and the others dominate, but add the smallest amount of the missing one and we tend to fixate on it's discovery, much as a lone figure in distant heat haze garners all the attention.
How does it make you feel? Is it more removed and distant, more interesting or less. Is the graphic nature diminished by the hint or colour "candy"? To me I think of Scandinavia. Can't tell you why, just a triggered memory I suppose.
Something I always feel with mono images is, they tend to look more evenly lit, more equitable in representation of depth and tonal range.
What if the image is manipulated, against it's true nature?
Does this feel warmer, or does it offer an untold story of sand storms or smoke filtered sunsets or even "old" styled sepia tone images?