Press UP or DOWN on the D-pad or Left Stick on the Character Select screen to change color to 2P color (or back to 1P color).
The following songs have unlockable harder versions:
Rambler
Days Dreamer
TGM in the Bottle
Clip Click
E.H.M.Y.
Stronger
Roteen de Moon
Pacmania
If you play any of these songs in arcade mode, you'll be able to play the harder version of that song as your next stage. After you play the harder version, it will be selectable as a normal song in Free Mode.
In Arcade mode on Normal difficulty, pass PacMania on your second stage. PacMania Hard Ver. will appear after Galaxian3 on your final stage.
Clear Arcade Mode once on any difficulty with any character to unlock Bonus Mode.
Clear a song in Arcade Made to unlock that song for play in Free Mode.