: If these colors all have the same lightness, then why does my brain tell me some are lighter than others? My human brain has a certain sensation of how "bright" a color is. More formally

@Hamm6457569

No, your brain does not have the dumb.



The eye responds better to certain colors than others, so some colors will always appear darker than others at the same relative (to 100%) brightness. This is something graphic designers, set designers, movie makers and lighting directors have to work with constantly.

Desaturating/grayscaling an image is a quick way to pull color out of the equation so you can look at the relative contrast in a layout. This is not the same as a Black and White conversion, which applies offsets to different colors to mimic film or various filters.

A design can sometimes fail because it lacks grayscale contrast, even though the colors are contrasting. When a layout will be seen from a distance or in small scale, tonal range (which is what "grayscale contrast" is) becomes very important to legibility because our eyes respond first to lightness (relative to 100% for a given color) and secondarily to color.

If something looks too bland in grayscale, color is not likely to salvage it.

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@Bryan765

i decided to test Scott's theory, that it's an optical illusion.


Really the colors are all the same lightness, and if i compared them side-by-side i would see they're the same lightness.


Starting with the "darker" blue from my original question, you can see the vertical gradient provided by Photoshop. That gradient is:


that is the same "color"
but with different lightness:




And the same with the "lighter" purple:



i take a single strip of that lightness scale from both colors, and overlay them on top of my original "50% lightness" gradient:




at the top, the bars are "too light" compared to the background
at the bottom, the bars too "too dark" compared to the background:




which means somewhere each line must have the same lightness as the background:



And in my opinion the blue is the same lightness at 50%, as the purple is at 50%, as the background is at 50%.

So they really are all the same lightness.

My brain is just dumb!

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@Murray976

Because our color perception is subjective/psychological and doesn't correspond to any mathematical models. Also, the LAB colorspace can't be produced on RGB monitors. In fact, many LAB colors are purely imaginary.

Furthermore, CIELAB is considered the most basic of the CIE "color appearance" models, which have been gradually developed over the years to better model perceptual color. Newer color models include S-CIELAB and CIECAM02, both of which use human contrast sensitivity functions to better model human color perception. A more recent and even better perceptual color model is called HCL. It was developed outside of CIE by Canadian researchers in 2005 and improves on existing models using a new similarity metric called DHCL.

See the wikipedia articles on color, color theory, and color vision for more details. This short document also describes the strengths and weaknesses of CIELAB.

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@Nimeshi706

Goes back to this illusion.....



The squares marked A and B are the exact same shade of gray.

Surrounding values alter the human perception. One reason why a neutral grey desktop is very beneficial when doing color critical work.

I don't know all the science behind it. I expect it would take a medical degree to fully understand.

(photo from web.mit.edu/persci/people/adelson/checkershadow_illusion.html)

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