I hear blue/brown etc, what exactly is the difference between all these?

Google.

The force in which the key registers in the computer and the noise they make kinda sorta not really.
Aka
Some are loud as fuck and some aren't.

http://www.overclock.net/t/491752/official-mechanical-keyboard-guide

This post was edited by Weird0ne on Jan 24 2017 08:25am

Actuation force and feedback.

Basically, how much pressure to get a keystroke to register and what happens as you press the key down. Some have a click, some have a bit of a bump, and some are linear (no feel).

I prefer lighter actuation and linear switches. People who type a lot like the bump... I think. Not sure why anyone gets clicky clacky keyboards. Maybe they just like making everyone in their house furious? Not sure.

The 3 most popular MX switches are Red, Brown and Blue
Red are the most silent of the 3, it will have a linear travel
Brown gives a tactile bump when it hit the actuation distance, not as silent as red, but a lot less noise compared to blue. But you still get that feeling in your finger and in your brain that you actually made the command.
Blue is one a lot of gamers prefer, i gives the tactile bump but as well a noise click.

They are different versions of these with a different actuation force. (how hard you gotta press the key)

You kinda gotta test them to know what you personal preference is. If you got a girlfriend sleeping beside you while you game red or silent keys are the way to go.

how do you come up with that those are the most popular?

Probably because most companies that use MX Cherry switches primarily have Red, Blue, and Brown models.

what about blacks they are used in all areas, industrial business and personal

lol

What incendiary wrote. And I know how to use Google :b