OK, so here's a serious one for the science geeks amongst us
For light, the primary colours are red, blue and green.
For pigment, the primary colours are red, blue and yellow.

I can handle the fact that they're different - one is transmissive the other is reflective; one is additive, the other subtractive. But what has always baffled me is why they are partly the same and partly different... All the same or all different, fine, but 2 the same and 1 different? How does THAT happen?

The universe is shoddily designed, if you ask me.
(moon monkey is busy making memories worth repressing, Tue 18 Oct 2011, 11:55, 15 replies)

primary pigment colours are also known as artists primaries and are Cyan magenta and Yellow
In printing one of the CMYK colourspaces is used, K being key or Black

Blacks obtained by CMY combinations depending on the print inks may be just a dark grey and similarly many dark colours won`t be dark enough.
Have carted out a definitive guide to light and colorimetry not available on line and will try and reduce and reproduce what it says as fast as I can type.
(tootired Is searching for a funny sig, nothing good so far, Tue 18 Oct 2011, 12:22, closed)

Ok here goes
you get that additively green and red= yellow, g+b= cyan B+R = magenta?

cyan dye absorbs red
yellow dye absorbs blue
magenta dye absorbs green

add Y an M dyes and you get red
ye= -B
M= -G
so RGB -B -G =R

Y+C
C=-R

RGB-B-R =G

C+M
RGB -R -G= B

www.optique-ingenieur.org/en/courses/OPI_ang_M07_C02/co/Contenu_05.html

might be clearer visually. My text that this was quickly taken from had wavelengh graphs and lots more from a course unit, forgive the scruffy.

It isn`t as simple as all that google trichromatic matching, ( tristimulus values, colorimetry matrices to get some idea.

Edit: there is a basic problem which is that the colour receptors in the eye have ovelapping irregular responses. in some cases given three suitable illuminants you can match them to tickle the eyes receptor exactly to the same value of a particular colour and brightness.

However It all gets a bit difficult. Monochromatic single wavelength excitation in cyan at about 480 nm will produce responses in the blue and green eye receptors and virtually nothing in the red. If we try to match that with a blue and green illuminant at at about 460 and 500 to match the eye`s peak response the green primarry stimulates the red receptor so that the colour looks too red. If using a visual matching colorimeter you need to add red to the primary cyan to match the two RG primaries. or to put it another way you have to have a -Red stimulation....WTF? In Tv systems you can matrix the sensor primaries so that the crosstalk between eye sensors is compensated by subtraction between the colour channels.

Typical daylight illuminance still leaves a few colours that can`t be resolved in the PAL or NTSC colour spaces for phosphors. This is a huge problem for LCD displays which is slowly being better addressed with LED back lighting, but there you go. the backlight should be a proper tristimulus tube like a scanner one and the transmissive dyes matched No idea how much more restricted the chromaticity triangle of subtractive dyes is.
(tootired Is searching for a funny sig, nothing good so far, Tue 18 Oct 2011, 12:48, closed)

Its all to do with colour models and colour mixing
I can't be arsed to explain, but google will tell you.
( Peej, Tue 18 Oct 2011, 12:25, closed)

'Google will tell you'
Is an apt answer to most of this week.
( scarpe We Stole Bikes, Tue 18 Oct 2011, 12:41, closed)

most of this week

generally
( Je suis un vagabond is an unfunny, up your own arse middle class knob, Tue 18 Oct 2011, 13:49, closed)

But they're not red, green and yellow.
They're cyan (blue-greeny), magenta (pinky-purple) and yellow. The first two look quite a lot different from blue and red.
(Me, I'm not... in any way intoxicated, Tue 18 Oct 2011, 12:31, closed)

And, on top of that,
different people perceive different colours differently.

Obviously.
(bat_fudgie, Tue 18 Oct 2011, 22:18, closed)

erm.. no only some
There was a horizon prog on this very subject, evolutionary evidence says blue yellow complementaries are hard wired, and evoke similar responses in the same brain areas, the later receptors do not provide exactly the same brain stimuli in different people. So yes to some colours and we have no way of confidently knowing for the others
(tootired Is searching for a funny sig, nothing good so far, Wed 19 Oct 2011, 15:17, closed)

Why
If you mix red, blue and green light you get white light. But If you mix red, blue and green paint, you get a shitty brown colour. WTF?
(Rimmer, Tue 18 Oct 2011, 14:08, closed)

At college...
I studied electrical engineering with a side of Lighting design for theatres.

Not only did i have the whole three phase colour system to master, but then the complexities of lighting mixes.

Headfuck? Often.
( Another_tragic_case wished he hadn't eaten all the cake, Tue 18 Oct 2011, 14:51, closed)

A headfuck is nice when you have your knowledge together and come prolifically in the mouth(s) of the chanutes causing it
I use the plural as many are 2 faced.
(tootired Is searching for a funny sig, nothing good so far, Tue 18 Oct 2011, 15:28, closed)

The difference is
'light' is , er light.

Colours are reflected light. They're what bounces off something, the light is not the same coming off of the surface as it was going in.

So, logically I guess primary colours absorb green light. Or something like that.
(username failed moderation, Tue 18 Oct 2011, 16:24, closed)

yes and NO
Visible coloured light is e/m radiation betwenn about 350 and 720 nm wavelength ( some people`s eyes do a little more and a little less but that is a very good approximation) reflected or directly received matters nothing.

Lay off the weed or stop being American because both encourage thickness of the head in already thick ones and smartness in the smart.
(tootired Is searching for a funny sig, nothing good so far, Wed 19 Oct 2011, 15:03, closed)

and
what kind of photoreceptors do you have in your retinas?
(haberman is singing in the changes, Wed 19 Oct 2011, 13:20, closed)

you have rods
and I`m not sure if it is politically correct to use the term "cones"
Edit they are reponsible for colour, so you see it is a photosensitive issue?
(tootired Is searching for a funny sig, nothing good so far, Wed 19 Oct 2011, 15:04, closed)