[Shadec (OP)]

what do you guys think?
im an adamant believer in the six colour theory, and constantly get into discussions about whether there is six or seven.
I have been told (by more than just one) by an optical physicist, no less, that the seven-colour theory is ridiculous. it originated when the man doing the 'scientific' analysis of the rainbow was a devout catholic, and that the number 6 was the number of the devil, man, sin and the like. so therefore, there must be 7, he just couldn't see it... infallible. unfortunately, this stuck.

Before i go any furher, let me give you a simple arithmetic lesson (you may need a calculator, but we'll see how we go).
There are THREE primary (im going with scientists colours) colours, Red, Green and Blue (RGB), and THREE secondary colours, Yellow, Magenta and Cyan.
it is generally accepted (now this is the part where most people get confused, so pay close attention...)
that:

3 + 3 = 6  [:0]

well... what do you think?

apparently 3 + 3 ≈ 7... somehow...

[lightarrow]

The number of colours is infinite.
The number of the colours you can perceive with the eye is not, but almost...

[Vern]

As lightarrow says, the number of colours is infinite. Colour is the frequency of the electromagnetic wave that comprises the visible spectrum. Your web browser uses three primary colours and mixes them to provide a less-than-infinite variety of colours. Each of the three primary colours can be one of 256 intensity values in your browser. 256 * 256 * 256 is white. 0 * 0 * 0 is black. So your browser can display 16,777,216 different colours. Or roughly sixteen million colour variations.

[lyner]

Not only is the number of colours effectively infinite but the number of possible primaries is also, effectively, infinite.  Different monitors, printers and displays will use different primaries because of the different technologies. They will be broadly RGB (or CMY) but not all identical, by any means. The received colour information needs to be 'bent' to fit the available colour space of the display as accurately as possible.
Remember, Vern, that your 255 value only refers to 24bit  digital colour coding.
And colour is not a "frequency". It's all in yer head. There are lots of colour sensations which cannot be produced using a single frequency. Your display uses primaries - your web browser (or at least the source encoder) uses 'indexed colours' which are a subset (sometimes a very small subset) of the 'millions' of colours which your 24bits can code.

[Vern]

Yes; and I didn't mention the alpha channel that we use to indicate the degree of transparency a colour might have. That adds another 8 bits so the colour is then 256 * 256 * 256 * an alpha of 256. That uses up the 32 bit width of a lot of processors, although now we mostly use 64 bit processors. And I have to type fast, or the number will go up before I can post the reply []

Colour TV with its three dots per pixel probably started it all. 256 just happens to be the amount of discrete values you can indicate with 8 binary bits.

[lyner]

Colourimetry will have started before TV. Colour film has been around for much longer and has used both additive and subtractive colour mixing. It's much harder to get things anything like right when all you have is buckets of chemicals and a thermometer - compared with the variables you can control in electronic signals. As with many things - the basic theory was done ahead of the technology.

The 8 bits per channel was a bit of luck, really. The eye doesn't readily resolve steps of 1/256 in R,G or B. If we had more discerning eyes then we'd have had to use 12bits per channel even for displays. As it is, 36 bit colour is needed when you intend to play with gain and 'curves' on the initial image.
(You young chaps-  computers used to use 16bits at one time, you know, and still did digital colour processing; it was just more clunky)

[syhprum]

I think it was Sir I Newton who first published results of splitting sunlight into colours with a prism, he was well known as a believer in Alchemy and his publishing on the subject exceeded his scientific writings.
He considered there should be seven colours instead of six for various mystical reasons, most observers would see six distinct colours although scientific analysis shows that the eye can resolve up to 2^36 subtle variations