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:
The fist one is to 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 :-)