In addition to the help article describing the general principle of adding colors , here you can find how you can define those colors in terms of how a browser would show them. On the execution tab of FDM and the Post-Production tab of the other processes, you open the color drop down.
As you can see, left of the name of the color there's a small round label with the color itself. Once you've added this color, it becomes clickable:
For the time being (13.03.2017) we have a small bug: The name "black" is showing the color white. There's a number of ways to solve this, which will work when you are creating colers from scratch, too.
Fist one is point on the part of the color field that's closest to the color you want:
Another way is to enter the screen color value directly in the field below, in hexadecimal values, #000:
In RGB: R=0, G=0, B=0:
or in HSL:
I myself use RGB, because that's one of the systems Microsoft Paintworks in (the other one is HSL): I open a screen print in that program and use the pipette tool to pick the color I need, and have a look at the description.
Once you've added the color and saved it, it will show up in the widget:
The absolutely same procedure allows you to add dyes to models in, let's say, SLS, but in Post production:
Don't forget to save when you're done :-)