I am not sure how to do this through the user interface, but you can change it with a script:
Names default to here(1);
dt = Open( "$Sample_data/iris.jmp" );
gb = Graph Builder(
Size( 531, 456 ),
Show Control Panel( 0 ),
Legend Position( "Inside Right" ),
Variables( X( :Sepal length ), Y( :Sepal width ), Overlay( :Species ) ),
Elements( Points( X, Y, Legend( 4 ) ) )
);
//Use XPath to find all LegendBoxes (there is only one), pick the first
//one that is returned, and set its text color to red
(gb << XPath("//LegendBox"))[1] << Text Color("red")
That said, I'm not sure why yours defaults to white, here is the chart I get when running that script before setting the text color: