IntelliJ - pink underlined variable... what does it mean?

For the first thing you're using lambda which is syntactic sugar. At the second thing the purple underline appears if a variable isn't from inside the lambda to show the user you're not using a local variable.

For example:

public static void main(String[] args) {
    int a = 0;
    Consumer<String> stringConsumer = foo -> {
      foo += a;
    };

    for (int i = 0; i < 1; i++) {
        String s = " " + a;
    }
}

If you paste that you'll see that only the a in foo += a gets purple underlined because it's from outside the lambda.