eli5 what color cards are in photography and what they do?
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Human color vision is not just we detect light and use the color of it directly to get the color. We compensate for the amount of light, the color of light if it is in shade or not, and what is around it.
Look at [this image](https://en.wikipedia.org/wiki/Checker_shadow_illusion#/media/File:Checker_shadow_illusion.svg) The square marked with A and B have the exact same color. Look at [his variant with single color bars](https://upload.wikimedia.org/wikipedia/commons/a/aa/Checkershadow_double_med.jpg) and block out what is beside them and it is quite clear the color is the same. Another way is to remove everyone else in an image editor.
Your brain is removed from the effect of the shadow the green cylinder cast and shows you the color that area would be if there was not shadow. It more exactly tries to do that.
The light object reflect to you is not the same on an overcast day compared to days with no clouds, you are not noticing it. If you take a picture with the exact same camera setting and then display them side by side on a screen it will look like different colors in the image. If you adjust you will not be the same as if you were there in reality, it is similar for the two images, and the result is the color look different
The reason cameras usually do not show this is because cameras have a white balance. They try to analyze the image and then adjust the color to make it look good. The simplest way it to assume the image is assumed to be on average gray and change the relative brightness of the color to get that. It often works very well but is not always. If on average gray assumption is not correct like if there is a lot of one color it can make mistakes.
[https://en.wikipedia.org/wiki/Color_balance](https://en.wikipedia.org/wiki/Color_balance)
There are setting on cameras for different common light conditions and you can also set the white balance on one object like a gray card, and then take an image of another object with the same setting.
Images with a lot of snow tend to result in a blue tint in the snow [https://www.ephotozine.com/article/why-s-the-snow-in-my-shot-blue–17912](https://www.ephotozine.com/article/why-s-the-snow-in-my-shot-blue–17912) It is not uncommon that digital camera has a snow setting for this reason
A color card has well know colors selected to make the color adjustment easier. You can adjust the color balance later in a computer and fix any error of what the camera did. You can lose capture a RAW images that are just sensor data with no balancing applied and do that at home. You can then use the color card as a reference and just the image so it has the correct color. You have now adjusted the image so it looks on the computer screen like how you would experience it in reality,
The [blue/black or white/gold](https://en.wikipedia.org/wiki/The_dress#/media/File:The_dress_blueblackwhitegold.jpg) dress that went viral in 2015 is an example of color balance. The color you see the dress depends on your interpretation of the light condition it was taken in. The dress is really black and blue. If you see it as white and gold your brain assumes blue-tinted illumination. If you see it as blue and black you assume a yellow-tinted illumination.
[https://en.wikipedia.org/wiki/The_dress](https://en.wikipedia.org/wiki/The_dress)
If you would have a color card in front of the image with no colors we could just the image so they look correct on the screen. You could then use it to change the color balance and the result is something like http://shuttersweets.s3.amazonaws.com/images/articles/scientific-proof-that-the-dress-is-white-and-gold/06.jpg
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