Tank Optics Help Needed Please.

So, I've been working on my own personal BF2 mod for about 4 years now. I'm presently upgrading the vanilla vehicles with new skins and huds.

I've made new huds for aircraft based on their real life counterparts, and now I'm moving onto tank optics, but I've run into a problem.

For my tank optics I want to use black on transparent as opposed to the classic green on transparent of aircraft huds, but I'm not able to achieve it.

I took a look at PR vehicle optics and see that they are, correctly, black lines/letter/numbers on transparent.

How do I achieve that effect? Do I need a particular type of alpha mask?

I'm using paint.net as my graphics editor.

Thanks in advance, Gatto.