How is the color integer calculated?

So expressions like rgb() and ColorValue return an integer number that can then be used to color something via an action.

That is useful for me as my game has a parser that uses pure text to generate various items on screen. The source text contains colors in hex so I'd like to write a function that would convert these to the C3 format and then the other way around. But how is this number calculated from rgb values? The C3's color format does not seem compatible with any color-to-whatever calculator I found.

Thanks!