Ikaruga is a vertical shoot-em-up ported from the Naomi Arcade version, and is more or less a sequel to Radiant Silvergun by Treasure, on the Sega Saturn.

Based on a two-colors system, the player has to switch between the black & white sides of his ship during gameplay. The black side is immune to black enemy's bullets which it considers as a bonus, and dies on impact with a white enemy's bullet. Your own black bullets damage white enemies twice as much as white bullets, and vice-versa.

This aspect, combined with specific enemy patterns and a combo system makes Ikaruga an original shoot-em-up with a dose of strategy.

 - From MobyGames