At the title screen, press Right, Left, Right, Down, Up(2). Then, change the number that appears on the upper left corner of the screen to play as different characters.
Wait until the message window appears while playing a game. Then, hold Select + Triangle + L1, L2, R1 or R2. While those buttons are held, press Up or Down to change the corresponding color value. L1 controls red, L2 controls green, R1 controls blue, and R2 controls all three values.
If you were looking for T: Story Begins With T for PSX cheats, then you might also want cheats for: