You can use change the text color by using Text Color() before text, for the border one option is to use Rect
Names Default To Here(1);
New Window("Example",
Graph Box(
Text Color("green");
Text(Center Justified, {50, 15}, "centered");
Text Color("blue");
Text(Center Justified, {50, 20}, "centered");
Text Color("red");
Text(Center Justified, {50, 25}, "centered");
Pen Color("Black");
Rect(40, 30, 60, 14);
)
);

I'm not sure if Fit Y by X lets you add custom fits but you could take a look at properties to see how those have been built

and try to imitate that
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
obj = dt << Bivariate(Y(:Weight), X(:Height));
obj << Fit Line;
Report(obj)[ListBox(4)] << Append(
H List Box(align("center"),
Popup Box({"",1 }),
//Spacer Box(Size(15, 0)), // to leave empty instead of red triangle
Spacer Box(Size(22, 2), Color("Orange")),
Text Box(" myfit")
)
);

-Jarmo