I really, really think they're more than readable enough now. Keeping in mind that they'll actually be moving and animating in the game, I think they'll be pretty hard to lose track of. Besides, I played Contra: Rebirth a few years ago, and it was one of the most fun games I've ever played, in part because the enemies were hard to keep track of once in awhile, with all the explosions and such on screen. It added to the challenge for me, and never felt cheap. Sometimes, you had to kind of visually map out where the enemies were, since they'd be behind explosions or foreground elements.
So I don't think enemies need to be lit up like neon on the screen. Most players are pretty forgiving of things like that, I think, and it actually adds to the fun if they're not completely invisible/lost on a noisy background. Which, in this case, they're not. I can easily spot them on the screen, even when they're not moving.
Not that I'm arguing against Corinthian's suggestion of upping the contrast on the sprites, per-say. I just think they're readable enough now, and any changes to them should be for aesthetic reasons - they might actually *look* cooler in a bright blue color, in which case, I'd agree with that change, too. =)
