At the suit selection screen in tactics battle mode, highlight a suit, then hold L1 + L2 + X, R1 + R2 + X, or L2 + R2 + X to alternate the suit color.
Select tactics battle mode. Select the Federation or Zeon, choose any mission, highlight the original Gundam or Zaku II then hold L2 + L1 and press X (no panels); hold R2 + R1 and press X (real type or desert type); or hold L2 + R2 and press X (blue remake or different colors).
Choose either Federation or Zeon, then choose Mobile Suit GM or Gouf on Zeon side. Hold L1 + L2, R1 + R2, or R2 + L2 and press X to get either a dark color GM or Gouf custom.
Use the alternate costume on the Zaku2 and you will play as the original Red Comet.