Describing a color

[agrahamt]

I am composing a FAQ to accompany a historical map for which I've added water color as an overlay. The color I've chosen created is rather arbitrary, but I am curious how to describe the color effectively in language. The overlay color is a shade of cyan, but it is 33% transparent with a yellowish 16th-century paper underneath, the combination results in an effective green.
I don't even know how to effectively describe the cyan - RGB numbers, but what about brightness? All the available characteristics in Photoshop's color picker are a bit confusing. Does LAB contain brightness information?
I don't want to get too technical (it's aimed at humanists), but want to describe it well.
I've attached an image also containing the solid color of the water overlay at 100% opacity.

Thanks,
Andrew
ps - here's what I have in my FAQ now FYI:
The overlay is cyan at 33% transparency, which blends with the yellowed paper of the compiled map behind it to create the water color. The black etching is also superimposed as a separate overlay above the “water” overlay to keep the lines from being overly blue. The Municipal archives suggest that the original water coloring was green, but there is evidence in the map that it was more blue than green. Also there are gardens on the segment photographs that are clearly colored green.

[andrewrodney]

Re: Describing a color

CMYK and RGB are device dependant color spaces meaning a set of numbers defines a color based upon some device. But Lab is based on human perception of color so use that. And yes, the L in Lab is Lightness which is a bit different from Brightness but since you don't want to get too technical, close enough.

Color, is a perceptual property. So if you can't see it it's not a color. Color is not a particular wavelength of light. It is a cognitive perception, the excitation of photoreceptors followed by retinal processing and ending in the our visual cortex, within our brains. As such, colors are defined based on perceptual experiments. Those experiences in the 1930's created the color models upon which Lab is based.

Cyan is an English word used to define these sensations!

[klev]

Re: Describing a color

LAB contains brightness information in the L channel. It is not intuitive though for color correction. You won't find many changes to be "perceptually" uniform, thus making it rather unintuitive.

[InfinityWork3]

Re: Describing a color

HSB is broken into the following:

Hue = the 'color' itself
Saturation = the amount of pigment of the given hue
Brightness = the amount of lightness on this scale: 0=black, 100=white, anything in between is a shade of grey

[agrahamt]

Re: Describing a color

Thanks all - really great information!
It sounds like LAB has more colors and device independence, but really if I'm web-publishing, that's not really device independence anyway. I might stick with RGB and HSB anyway (aka ctrl+u!), will think about it and add to thread at some point. I wonder if it's worth mentioning srgb color profile...

Best wishes,
Andrew Taylor

[andrewrodney]

Re: Describing a color

Lab doesn’t have 'more colors' but it has a pretty wide gamut. The two are completely separate. A 24-bit file has a fixed number of possible colors due to the encoding. But the gamut can be vastly different.

There are thousands of recipes of RGB. So you can't just say 'RGB' without a color space being associated with those numbers. If you provide numbers and say sRGB, the color has a meaning now (and that's all based upon Lab).

[John Wheeler]

Re: Describing a color

.............. I wonder if it's worth mentioning srgb color profile...
Best wishes,
Andrew Taylor

Hi Andrew
Absolutely you need include the color space (echoing what Andrew Rodney said). A color space among other things represents a color scale.

Here is an analogy. If someone said it was 50 degrees out one might bundle up if they were accustomed to degrees in Fahrenheit. However if you were accustomed to degrees Celsius then at best you might want to be in a pool (50 degrees Celsius = 122 Fahrenheit.

As a visual example (displayed in sRGB so it can be viewed with virtually all monitors), if you plug in the color numbers into various color spaces (no color management) you would see the colors below for the different color spaces as shown by the text in the image. The one on the right is the icc profile of an Epson printer using Premium Luster Paper (its color space/scale). Clearly just specifying the numbers does not tell you the colors you eye will experience without tagging on the color space and the differences are not small in the colors that get created. Click on thumbnail to enlarge.

Hope this incremental information is helpful.

[andrewrodney]

Re: Describing a color

Here’s another metaphor for a color space: suppose I supply a recipe for chocolate chip cookies but do not provide the unit for each ingredient in the recipe. The recipe provides each ingredient followed by a number. Without units you can’t make the cookies. The numbers alone are not enough information to describe how the cookies should be produced.

Likewise, R78/G103/B23 or C23/M98/M123/K6 is not enough information to reproduce that color.

Going back to the chocolate chip cookie analogy, suppose a color model is a cookie recipe with only three ingredients. I give you this recipe, which simply calls for 1-flour, 8-butter and 2-chocolate chips. You don’t have enough information to make the cookies. However if I provide you the recipe with a specific scale—1 cup of flour, 8 tablespoons of butter, and 2 cups of chocolate chips—I’ve provided the necessary information, the scale, to make a dozen chocolate chip cookies. I can give you the cookie recipe in the metric scale such as liters and grams and you can still makes the same cookies even though the numbers are different.

A color space is a color model that has a known reference and scale, in this case primaries (the ingredients, in this case RG and B) and scale (specific quantities of these ingredients).
Suppose I specify a color as R10/G130/B50 and specify a color reference by saying the color space is Adobe RGB (1998), which defines the scale of the RGB primaries; the color coordinates of this color space. The R10/G130/B50 set of numbers can now reproduce a color by anyone with the proper tools since the reference and scale have been defined.

Different RGB color spaces use a different scale of red, green, and blue primaries. Adobe RGB (1998) and sRGB are different color spaces, however both are based on the RGB color model using RGB primaries. Although each color space uses the same three primary ingredients (R, G, and B), the specific colorimetric scale of each color space is different. The maximum of red, green, and blue are more saturated in the Adobe RGB (1998) color space than the sRGB color space. Even though R0/G255/B0 is the greenest green ingredient in both Adobe RGB (1998) and sRGB, knowing that the scale is different in both color spaces explains why this green value is more saturated in Adobe RGB (1998). This also illustrates how R0/G255/B0 alone can’t tell us what green.

An ICC profile simply defines this scale and gives the numbers a meaning allowing us to reproduce the color using something far more concrete than using the English word "Green" or a set of numbers which alone is far too ambiguous to produce a specific color appearance.

So R255 in Adobe RGB (1998) and R255 in sRGB share the same numbers but have a different scale. When you assign a color space to either number, you’re telling Photoshop the scale, it’s updating it’s preview and showing you this (correct or not, you’ve defined the scale so Photoshop believes you). The number hasn’t changed the color appearance has. Now you know why.