Variant #1:  What color is the grass at night-- green or black? 

Is an object's color defined by the wavelength of the light it reflects in 
ambient lighting conditions, or is the color defined by the light it reflects 
when bathed in a source of white light?

Variant #2:  Am I really a white man by day and a black man by night?

Your question can be approached in different ways. If you wish to view “color” 
as a property of the surface of a particular object, independent of a human 
viewer, then your first definition is consistent with common scientific 
descriptions of color. Color depends both on the illumination and the 
reflection characteristics of the surface. Objects can change color with 
illumination. You can see this readily, for example, in the apparent change in 
color that occur when you walk under sodium vapor streetlights. A more subtle 
demonstration can be made in comparing the appearance of certain paint or ink 
colors (pigments and dyes, generally) under, say, fluorescent, incandescent and 
daylight illumination. There are certain “metamerism” effects where two colors 
will appear identical in one kind of illumination and different under another.

The situation is really more complex than that, however. Perceived color also 
depends on how the eye and brain process the light coming into the eye. There 
are, for example, additional apparent color changes that can occur as a 
function of the color of surrounding objects as demonstrated by the 
famous “Mondrian” experiments of Edwin Land.

But, I think the correct answer to your question really has more to do with how 
the perception of color changes in low-light conditions. What happens to the 
perception of color as you reduce the illumination level without changing 
anything else? (“Night” implies [at least to me] low light, not no light--there 
is almost always some light.) This brings us to how “night vision” works in 
humans. The human eye distinguishes color using a set of three receptor types 
(cones) with peak sensitivities in the red, green and blue respectively. These 
are used to see color under bright ambient light. They provide good spatial and 
color resolution, but are not sensitive enough to provide useful vision at 
night. There is a separate set of receptors (rods) with much higher light 
sensitivity. There is only one set of these in the human eye with a peak 
response in the green. The net result is that as you lose the information from 
the cones and use only the rods to see, the brain can no longer distinguish 
colors. Since this is an everyday phenomenon, most people don’t interpret the 
change as a change in color. They may not even be aware that color information 
has been lost. They just know it’s dark. If pushed, though (say, by 
artificially constructed tests), people will usually describe low-light scenes 
as having no color at all. The scene becomes only shades of gray. Thus perhaps, 
the best answer to your “Variant #1” is that the color of grass at night is 
neither green nor black, but rather gray.

The answer to “Variant #2” similarly depends on a definition of terms. If 
by “white” and “black” you want to identify racial groups, then, in most cases, 
you can argue that the identification would not change. There are enough 
characteristic racial features besides skin color to continue to provide good 
cues for racial identification in low-light conditions (although 
misidentifications in ambiguous cases may become more numerous). If you mean 
specifically skin color, you may have more difficulty distinguishing small 
differences, but you would still be able to distinguish easily between the 
color of, say, a very pale Nordic hand and a deep ebony Central African hand, 
because the relative reflection of light from the skin compared to surrounding 
features (such as clothing) would still be noticeably different.

For further reading on the fascination subject of color vision in humans, I 
suggest the following on-line resources:

“The Human Eye,” by Dr. John W. Kimball
http://www.ultranet.com/~jkimball/BiologyPages/V/Vision.html
(good general discussion of rods and cones and how the eye works in general)

Color Science (a web page from IBM Research)
http://www.research.ibm.com/image_apps/colorsci.html
(describes metamerism in detail)

“Measuring the colours we perceive,” by Daniele Marini and Ludovica Marini, 
Science Tribune, October 1997
http://www.tribunes.com/tribune/art97/mari.htm
(a good introduction to color constancy and color illusions including Land’s 
experiments)

Bonzo,

An interesting question. However, aside from the technical explaination (that 
you have already touched upon) it eventually comes down to an individuals 
perception of colour.

As a rule, it is likely that the majority of the populace would believe that 
grass is, indeed, green, there are exceptions 
(http://www.geocities.com/HotSprings/8018/Defects.html) and those who think 
laterally (a term coined by one Edward DeBono [http://www.edwdebono.com/]), or 
question reality (http://www.heise.de/tp/english/special/vag/6037/7.html) may 
think differently.

Other sources for the nature of colour:
http://www.colormatters.com/
http://library.thinkquest.org/13405/color/what.html

It doesn?t have any colour actually.

The cells in your eyes are not able to function right in dim light conditions.
So we know the grass is green because we have seen it in daylight. But in the
night their is not enough light to make our eye cells perceive the colour.

In dim light conditions your eyes can only perceive outlines of objects and
cannot sense the colour.

That is why the grass doesn't have any colour in the night.

My girlfriend has a dress which shimmers from red to green, depending on the 
angle it's viewed at, the light source, and so on.

What colour is it *really*? Red or green?

John Locke - the seventeenth century philosopher - argued that objects have two 
different classes of quality - primary and secondary. Primary qualities are 
those which are fixed, like size and movement; secondary qualities are those 
which depend on perception - the smell of an object, its sound, its colour and 
so on.

Different people may perceive these secondary qualities in different ways on 
different occasions - if I have a bad cold, for example, a rose will smell very 
different. Similarly, in a darkened room I might conclude that the grass, or my 
girlfriend's dress, is pitch black.

Locke's conclusion was that objects do not in themselves possess these 
secondary qualities, but that they are produced by the fact that we interact 
with the objects. 

The grass, then, is neither green or black - it has no colour, but it *does* 
have the power to produce the effect, if it comes into contact with a viewer, 
or producing greenness or blackness.

An excellent two volume set of recent philosophical and scientific studies of 
color:

http://www.amazon.com/exec/obidos/ASIN/0262024241/qid=1019338480/sr=8-
1/ref=sr_8_1_1/104-9555274-9263158

Hi Bonzo, 
         This reminds me of the old Zen Koan "If a tree falls in the forest, 
and no one is there to hear it, does it still make a sound?" but your question 
is an interesting twist in that it is the light that is changing, not the 
presence of a person.

Sir Isaac Newton (referring to his experiment in splitting light with a prism) 
said that: 

"For the Rays to speak properly are not coloured. In them there is nothing else 
than a certain Power and Disposition to stir up a Sensation of this or that 
Colour." (Sir Isaac Newton, Opticks, 1730)

See 
http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_15/ch15p1.html
for the full quotation, and a lot more. 

According to this view, grass, or the light it reflects, is not coloured - 
colour is our interpretation of light. We can never be sure of how another 
person perceives a colour and a colour-blind individual obviously has a 
different range of perception from ourselves.

In fact, certain flowers have some visual patterns which cannot be perceived by 
humans at all but are attractive to bees. So do those patterns exist, or not?

See http://spectacle.berkeley.edu/class/opt10/moy01.shtml
for a nice comparison of a flower as seen by a human and by a simulated bee.

It fully depends on how much light there is and what its source is,
but if you're looking at grass by the light of the moon, I'd have to
say it was 'blue'.

The color sensors in your eyes can see BLUE best in low light
settings. With low light, the entire environment would appear bluish -
ever notice how they use blue filters on TV to simulate 'nighttime'?

there is an island in the ocean with a palm tree on it, nobody has
ever been there, what colour are the leaves? the answer is they have
no colour as for colour to exist as a concept you need 3 things a
light source, an object and an observer. in the above case there is no
observer. your example tends towards there is no or litle light
soursc, therefore there is no or little refeltance of the incident
light and so the object is corresondingly dark or invisible.

An island in the ocean where NOBODY has ever been?

Wow!

Must visit.

Could somebody provide the co-ordinates please?

Does this relate to the question:

"If a man alone in the woods speaks,  and his wife cannot hear him, is
he still wrong?"

http://www.physics.utoledo.edu/~ljc/wife.htm

:-)

ts

to quote:

" Applying the psi function, the more vague the statement of the man,
the greater  the probability of his being correct. The narrower and
more specific  his utterance, the greater the likelihood of his being
wrong.  Also, the principle of complementarity assures us that if a
man  alone in the woods speaks, and his wife can not hear him, he is
BOTH right  and wrong--until he comes out of the woods."

keep this in mind, guys . . .

ts

Pro,

that island exists somewhere between the hypothalamus and ye olde frontal lobes

nice place to visit but  . . .

ts

The color (reflected photons) is dependent on the wavelength of light
that is the illumination source so the colour is what it is when you
see it. on a planet with a blue sun the colour would be different than
on earth, so the context of the colour is also in question. On earth
grass is green during the day and black at night in the abcence of
illumination, On PX136 my home planet Grass is pink. :-)