system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why digital colors are expressed in Red Green Blue (RGB) and not in Red Yellow Blue, the primary colors?
gpt
Because primary colors aren't, really. 

The human eye has three different types of cone cells that can each "see" a different color of light. These are roughly red, green, and blue. So, right there, you should be able to understand why colors on a screen are represented by red, green and blue; they're just tuning in to what our eyes can see. By mixing these colors of light, we can create all the other colors our eyes can see. 

So why are the so-called primary colors red, blue and yellow? 

Because we people started thinking about colors, they had no idea about how light worked, or how our eyes worked, but we did know something else: paint. by mixing colors of paint, you can get a different color. By trying a lot of combinations, people found out that with red, blue and yellow, you can make (almost) any other color, so the called those the primary colors. 

But mixing colors works a little different from mixing light. Paint has a certain color, because it absorbs all the other colors. So if you add different colors of paint together, the mixture gets darker. But if you mix different colors of light together, the mixture gets lighter (because more light!). 

Now that we know how light and our eyes work, we have learned that what we first called primary colors are slightly off. To be able to make all different colors using paint, you actually need 

* yellow (that one was right) 
* cyan (a sort of light blue) 
* magenta (a sort of pink) 

These colors are what you get when you mix red and green, green and blue, and blue and red light, respectively.