Couple of things I should really make clear:
1) It's a mockup (though those gfx do actually exist and get used in the game but it doesn't look like that currently)
2) It (at least when I started it) used a dual-playfield, so it only used a total of 32 colours for the actual game screen (16 for each playfield), and another 16 for the score panel. And another 16 will be used for the player's ship as it's a hardware sprite.
That's not to say it'll turn out like that as I'm not doing it anymore, at least for now.