One thing you might try...

In this example, the antialiasing color should be R:128,G:128,B:128 -- in other words, a medium gray, since the foreground is white and the background is black. Do some simple experiments with just that part of your code that determines the average color.

Try it out with different foreground/background colors where you know in advance what the answer should be, for example, if you had used the red background, the antialiasing color should have been R:192,G:128,B:128.

That's the way I'd proceed, anyway.

-Bob