Finally! Someone says something interesting! These crosshairs are meant exclusively for use with the crosshair gui, which allows you to pick and choose crosshairs for different situations. I would in no way recommend using these by themselves, and so if you want a more realistic or less interesting crosshair, there are plenty of those around, and I hope you go and get one. However, these are still good for different situations: the different colors work better with different environments. So there. I refuse to remove the green and "purple."
Oh, and to top that off, I think I'll go so far as to say that a one pixel white line would not help in the slightest. These crosshairs were made in GIMP 2, which is very good for creating smooth circles out of not very many pixels. These are 32x32, mainly because they work best with the crosshair gui that way. So if I added a white line that was even just one pixel wide, it would mess up the whole crosshair.
There is no other good way to create a white boundary at this point, either. And, in my opinion, no reason to.