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Great fun all round...
It's interesting that black-and-white is often called 'monochrome' - which covers coloured images with a single colour - like most forms of toning.
And you're right - strictly, black and white are not colours, as each white light is a mix of all colours, and black is an absence of all of them.
I think that you'll find that the only people really concerned are those who write rules for competitions, and each may have its own version of what is acceptable as 'black and white'.
Otherwise, the world is your oyster: even split toning (now available in Photoshop, originally requiring two different toning baths in the darkroom) tended to be acceptable in the days of film, as the image came from a black-and-white negative...
It's interesting that black-and-white is often called 'monochrome' - which covers coloured images with a single colour - like most forms of toning.
And you're right - strictly, black and white are not colours, as each white light is a mix of all colours, and black is an absence of all of them.
I think that you'll find that the only people really concerned are those who write rules for competitions, and each may have its own version of what is acceptable as 'black and white'.
Otherwise, the world is your oyster: even split toning (now available in Photoshop, originally requiring two different toning baths in the darkroom) tended to be acceptable in the days of film, as the image came from a black-and-white negative...

A true black and white image would consist of only black and white colours… But, what we call a black and white photograph is made of black, white, and many different shades of grey…
“Grayscale” is a more technical and a more accurate term for the type of photography we call black and white… The technical definition of “grayscale” is an image that is composed exclusively of shades of gray, varying from black to white…
You’ll find photographers use the term “Black and White” while those in the design industry favour the term “Grayscale…”
Usage of the term “grayscale” has increased since the use of digital imaging has taken over film photography…
The term “monochrome” has its origins in the medieval Latin word monochrōma… It consists of two words: “mono” and “chrōma,” which translates exactly as “single colour…”
If we define monochrome photography, we would define it as an image composed exclusively of shades of one colour varying from the brightest to the darkest hues…
The best example of a monochrome image is a sepia photo, which has its origins in film photography…
Grayscale vs Black and White…
In essence, “grayscale” and “black and white” in terms of photography mean exactly the same thing…
However, grayscale is a far more accurate term… A truly black and white image would simply consist of two colors…black and white… Grayscale images are created from black, white, and the entire scale of shades of gray…
Grayscale vs Monochrome…
All grayscale or black and white images are monochrome as they are made of varying shades of only one color…black… However, not all monochrome images are grayscale as monochromatic images can be made of any color…
An image made entirely of shades of yellow would still be considered a monochromatic image…
“Grayscale” is a more technical and a more accurate term for the type of photography we call black and white… The technical definition of “grayscale” is an image that is composed exclusively of shades of gray, varying from black to white…
You’ll find photographers use the term “Black and White” while those in the design industry favour the term “Grayscale…”
Usage of the term “grayscale” has increased since the use of digital imaging has taken over film photography…
The term “monochrome” has its origins in the medieval Latin word monochrōma… It consists of two words: “mono” and “chrōma,” which translates exactly as “single colour…”
If we define monochrome photography, we would define it as an image composed exclusively of shades of one colour varying from the brightest to the darkest hues…
The best example of a monochrome image is a sepia photo, which has its origins in film photography…
Grayscale vs Black and White…
In essence, “grayscale” and “black and white” in terms of photography mean exactly the same thing…
However, grayscale is a far more accurate term… A truly black and white image would simply consist of two colors…black and white… Grayscale images are created from black, white, and the entire scale of shades of gray…
Grayscale vs Monochrome…
All grayscale or black and white images are monochrome as they are made of varying shades of only one color…black… However, not all monochrome images are grayscale as monochromatic images can be made of any color…
An image made entirely of shades of yellow would still be considered a monochromatic image…

Malc's explnation is succinct and relates to physics (so isn't negotiable) and as Dave points out too.
It's only because we humans can perceive different wavelenghts. In fact, thge rods in our eyes are sensitive to light level only, useful in low light. If we were only sensitive to one wavelength it'd be white and shades of grey. We wouldn't know what 'colour' was/
IR film is blackand white because it just records the intensity of IR (specific wavelenght) only.
Colourless liquids are such because they don't absorb th part of the spectrum we are sensitive too. If we could see different IR wavelenghts how different our world would look. By the way, colourless is different to clear.
As for chocolate, it has to be dark
It's only because we humans can perceive different wavelenghts. In fact, thge rods in our eyes are sensitive to light level only, useful in low light. If we were only sensitive to one wavelength it'd be white and shades of grey. We wouldn't know what 'colour' was/
IR film is blackand white because it just records the intensity of IR (specific wavelenght) only.
Colourless liquids are such because they don't absorb th part of the spectrum we are sensitive too. If we could see different IR wavelenghts how different our world would look. By the way, colourless is different to clear.
As for chocolate, it has to be dark
