Addicting and stimulating!
ParagonX9 always does a good job giving us that warm, Newgroundsy feel of olde...years and years later, that song is still good, but I would have include an option for the player to choose between a variety of songs; it's strangely only a 1.5 MB game, so it couldn't take up THAT much extra room, could it?
I would also include a bonus for those who are accurate; if you hit, and hit, and hit, and hit, without missing, you can gain combos that intensify your score dramatically, or even unlock special, previously unobtainable perks...you could also see to Tom about implementing the achievement system in this game to unlocking achievements through NG accounts; some users like to collect those.
I quickly found myself opting for speed and rapid power favoring over a balanced health system...I stayed tiny, but when enemies started coming out in suffocating swarms, I quickly upgraded to rapid fire. When I blurred my vision, and kept everything peripheral, I could concentrate on the brighter shade of color as the enemy, and manuever in tight-patterns dodging enemies and blasting only what's nearby until they started to spread thin...I began dying rapidly around the color changing one's (white-to-purple-to-red each time you hit them) because that threw my strategy off.
...Oh, and that strategy also didn't work with the super-fast purple ones. :P
The strafing shooting enemies were interesting at first, but never presented a huge challenge. I kept glancing at my health to see if I were getting hit, and I don't think they landed a single shot on me after their first encounter. There needs to be more enemies with projecting attacks, or even proximity attacks to counter my close-up, or "chase-me" tactics.
The game is pleasing to the eye, and as distinct a noise the data collection "twang" was, it never got annoying for me. It all had a very good feel to it. Since this is an incredibly visual game, I would suggest enemies that could alter or play on that. One's that are two dimensional and seem to flip, to give the illusion that their visibility wanes off and on as they strobe their way towards the player...or flare orbs that emit and abundantly radiant light to cloak other enemies that could be hiding in the aura. Stuff like that wouldn't be too atrociously difficult to add, but could turn the tide of difficulty in the stage.
I think you're on the right track; I had fun playing this! Keep up the good work!