When does setState trigger a re-build in Flutter?

I am not sure exactly when setState triggers the rebuild in Flutter.

I have some text on a screen that I update with setState. It rebuilds nicely. Here is the code:

setState(() {
 futureWait = "$fifteen second wait for both futures completed";
});

So I wanted to change the color of the text as well so I made a Boolean that I set after the setState function. I did that like this:

setState(() {
 futureWait = "$fifteen second wait for both futures completed";
});
futureWaitBool = true;

The widget looks like this:

  myText(futureWait, 15,
        !futureWaitBool ? Colors.white : Colors.green),

It works fine for now – but I wondered if I created a race condition. Is there are race condition if I do this?

setState(() {
 futureWait = "$fifteen second wait for both futures completed";
 futureWaitBool = true;
});

I can easily use the Boolean to switch the color and string and in that sense make it atomic. But I am really asking how it works and where is it documented. The existing documentation is unclear in my opinion.

For example the documentation says: "Calling setState notifies the framework that the internal state of this object has changed … which causes the framework to schedule a build for this State object." The documentation is written as if just the fact that you call it sets in motion the notification to the framework. I want to know does it happen when
a) you first call it;
b) you actually change a state variable in the callback;
c) when setState completes;
d) or some other time.

Thanks in advance.