The letters correspond to their color. Y = Yellow, R = Red, B = Blue and so forth.

From the Main Menu, go to Options, and into the cheat menu.

From there enter:

YO, RB, RO, GB, RY, YO, RY, RB, GY, GY, YB, YB, YO, YO, YB, Y, R, RY, R, Y, O

You should get a confirmation that this code worked.

Note: The songs will go away when you turn your Xbox 360 off, so you'll have to enter it again when you turn it back on.