DEC colours

[Gordon JC Pearce]

Ben Franchuk wrote:
> der Mouse wrote:
> 
>>
>>
>> RGB isn't enough - not everyone's visual system's response can be
>> modeled accurately by RGB.  (Some people have colour receptors whose
>> response peaks are at nonstandard frequencies; a few people even have
>> more than three different colour receptors.)
> 
> 
> Just what kind of people have more than 3 color receptors.

Some people (all of them women) have four different colour receptors. 
There are two different kinds of "Red" cone, chosen genetically.  Yours 
may be different from mine.  We might well match two different Pantone 
cards against, say, a piece of cloth, or a switch on the panel of a 
large computer.

Now, the gene that codes for which type of red receptor you have is in 
the X chromosome.  One of the reasons why boys are many, many times more 
likely to be colour-blind than girls is that girls have two X 
chromosomes, so the "good copy" will be used.  What happens then is a 
bit more interesting - the retina of the girl's eye will have 
colour-blind spots.  You can find these by shining a very narrow 
coloured beam into the eye, and moving it about.  Some bits of the 
retina won't let her see what colour the beam is, some will.  It's 
exactly the same way that a black-and-white cat and a ginger cat will 
have tortoiseshell female kittens.

So, if one parent has one kind of red cone, and the other parent has the 
other kind of red cone, then the resulting retina will be a patchwork of 
both types of cone.  Women who have tetrachromatic vision can tell the 
difference between tiny gradations of colour, so small that most people 
cannot see it.

Gordon JC Pearce.