…so a blue thing of the same brightness as a grey thing will look darker than the grey thing…
I don’t know what the heck you’re seeing, but they look the same to me… (on a properly calibrated monitor)
So we’ve pretty much established that Jerommel has eyes sensitive to light in the Yellow spectrum, if indeed that is a given, it only means you’re more abnormal than we first assumed. 
Green is the color the human eye is most acclimated to. Look around, grass, leaves, virtually all plants use green to convert sunlight to energy through photosenthesis. We see green easier than all other colors. You may be sensitive to yellow and as such it appears extraordinarily bright to you, but that’s an anomaly, the exception to the rule.
The earth’s atmosphere converts sunlight to a normal primary daylight color in the 5700K range, which is why camera flash units emulate the 5700K spectrum. Daylight white, the ever elusive color of light in the shadows of a tree, is a primary light source the world over. It’s been argued that we as a human animal spent so much time around a campfire with some 3000K color temperature that we find the warm orange light comforting and natural. It may well be that your ancestors passed on an acuity to the colors you perceive as brightest, based on what they did for the better part of their day.
It’s a complex and sometimes convoluted equation, as with most things, especially women. 
By comparison, take the tritium vial which is illuminated by a radioactive isotope of hydrogen. This is a low output light that the source is as constant as most any we’ll find, with a half life of some 10 years. In the 9 commonly available colors, green is considered to be 100% output while yellow is 20% dimmer at 80% of the light emitted by green.
To the vast majority of people, the green color is brighter. There are, however, a great number of people with varying levels of color blindness that can’t differentiate between green and yellow. So the only constant is the variability of the human animal.
Well, we should agree blue is the darkest colour, and its opposite is yellow…
I don’t know…
But i would prefer a yellow LED over a green one when i want to see in the dark.
A pic to consider maybe:
Probably because you’re thinking of warm white LEDs which look yellow, but that’s still white light, you can still see colours.
If you had pure yellow you would not be able to see any colours. Same with pure green.
Again, the reason green is used in night vision equipment is because it is more visible and you can distinguish more shades of it.
PS. you’re right about blue:
No, i’m thinking low pressure sodium light, which is monochromatic yellow, or a yellow 580nm LED, not a 2700K white one.
But isn’t the colour in the colour circle above brightest at 2 1 o’clock (30°) to you?
When i grayscale it according to luminosity it is also the brightest.
To my surprise cyan is almost as bright.
Indeed.
I think the ‘problem’ lies in the difference between sensitivity and perception.
Open this in full res and then tell me which one looks brighter to you.
Jerommel, if you take two light sources with the same output power, one yellow (say 590nm), the other lightgreen (555nm), (even) you will notice that the green source appears brighter.
If yellow seems brighter next to green, it is because the yellow source has more radiometric power.
I think it very unlikely that there is a person-to-person variation, since the eye pigments are distinct chemicals with fixed absorption spectra.
There are a few rare alternative pigments present in the human population, very few people have them. (would have to look up what what that implies for the sensitivity spectrum)
I will if you answer my questions too.
Seriously though, it’s pretty hard to tell.
The green pixel seems brighter but it’s also smaller and thus more like a pin point, because it’s only the green segment of the monitor pixel, whereas the yellow one is both red and green, and both are slightly dimmed.
[EDIT]
On second sight, the green one indeed looks brighter.
Hmm…
That sounds like “radiates more”, which sounds like “brighter”…
I’m afraid i’m not gonna understand this…
The only way i can rationalise it is that it’s the difference between sensitivity (receptors) and perception (brain, our biological graphics card).
Yeah, the green is brighter.
And on your circle, it looks brighter at 12, not 2…
My bad, i meant 1 o’clock, but i did say 30°:
(grayscaled by luminosity colour circle)
I think we are “slightly” off topic :person_facepalming: , anyway I guess our eyes aremost sensitive to one precise color: Sun color! It was the only light source we had available for our first millions years on the Earth, so we are totally “matched” to its apparent color.
Sun color consits of “many color”, and UV, xray, and many invisible colors
Yes but we are talking about light which we can perceive.
Sunlight looks yellow.
Sunlight look like 5000k white at 3h pm at my place
Daylight is white.
Just a quickie from me, and then I’ll let it go. High pressure sodium isn’t monochromatic yellow. It does produce a spectrum of wavelengths centered around yellow, though. Here’s a picture from Wikimedia. 
The main reason HPS lamps are/were in use for street lights and other forms of bulk lighting is because they are cheap and efficient. I read recently that HPS bulbs are still more efficient than LED technology, even as other forms of lighting are losing that race to the ever-improving LED tech.
As for the conversation about which color is “brighter”, you need to stick to one definition of that word if you want to get anywhere. It is known by scientists that the human eye perceives green the best. So, if you have several sources of light emitting the same power, the green will look “brighter” to the eyes. But, on a monitor, the yellow “color” is actually a mix of green plus red, so it is going to be emitting more power, and may look “brighter” as a result of that extra power. It’s literally just like a XP-G2 looks brighter than a XP-E2 because it actually is emitting a lot more light. But, to me, white bulbs look a lot brighter than HPS lamps. I think “warm” lighting looks dim and dull.
An off topic reply :party:
The perfect adapted human body sounds logical, but biology and chemistry is not that perfect at all. Forget the idea of perfect adaption, biology is often about make do with what is available, with thee simpke rule: what works, stays. In case of the light perception in our eye: it is a most primitive system with three badly overlapping receptors, but it was what was available so it had to do. Luckily our brain has a great way of dealing with the little and incomplete information so we do not notice it normally.