..I was wondering how the HELL you expected us to beat the boss at Level 8.. Up until then, I didnt even know you could change ship colours, and it was pretty easy (I think I only lost 2 lives).. I eventually died on the level 8 boss, read a little more, found about the POLARITY ability, and went back with a vengeance.
That boss was by far the hardest one you come across.. the final boss was actually one of the easier ones. If you switched them around, it would be almost perfect difficulty adjustment.
I have to tell you, I really didnt like the fact that when you shot the upgrades, it changed them, and eventually blew up if you shot them enough. I constantly held down the spacebar, and noticed I didnt really care what the powerup was, I just went for it. It would be better if they were a set upgrade once they came on the screen and stayed like that, so you could choose if you liked that upgrade and was worth the risk to fly closer to the top of the screen to obtain. It was just too hard to remember which symbol meant which powerup.
Overall, though, it was pretty easy (once I found out the main gimmick of the game!) until level 13-16 (not including the last boss).. which got pretty ridiculous. I also thought the blue flying heatseekers looked more white, and at first glance, I thought they were unavoidable (as in, either colour ship wouldnt protect you), but then I saw the red heatseekers, and knew the white things had to be "blue."
Good game, albeit with a few flaws, mixed with a pretty cool idea.. nice work.
[I see you have several other 'long' reviews down below.. I guess it shows people care about your work and are willing to offer detailed help! =)]