Alot better way... Make the shield effect on a totally black background, as if it'd be drawn on some black space already. Then draw it additively. That way the black will be invisible, and brighter it gets, the more visible it'll be. (In fact, draw about everything additively that's gonna glow) Also note that this is a pretty slow thing. So if you will use additive drawing, be sure to have the latest UnDelphiX and Hardware Accel on...
You should use an alpha of 255, that way, the effect will look about like how you've drawn it.
(For an example, lookie at http://freespace2d.try.hu, screenshots section... Yeeaaa, that's coloured additive drawing, same thing )

Of course, everything runs faster in full screen than in windowed...

czar: now who's saying bad things about UnDelphiX? You or me?