For the visual feedback we developed a light concept which integrates several LEDs: single LEDs for the background illumination of the buttons, LED rings around the three buttons on the front side and an LED strip inside the handle. All LEDs we used are Adafruit's so-called NeoPixels which can show all RGB-colors and are individually controllable.
The light concept is depending on the different states of the player:
is ready (after boot, nothing is playing at the moment, waiting for a token to be attached): all buttons are yellow, the rings are off
is playing (after a token has been attached): the play/pause button is green as well as the ring around it, the next and previous buttons and the rings around them are blue, the recording button stays yellow
is paused: the play/pause button switches to red, the rest stays the same
pressing next/previous: the next resp. previous button turns violet while being pressed to go to the next/previous song and pink while being pressed and held to wind forwards/backwards, the rest stays the same
is ready for recording (after the recording-token has been attached): the recording button turns green, the rest stays as it was
is recording (after the recording button has been pressed): the recording button turns red, the rest stays the same
The LED strip is always mapped to the current volume setting: the louder the pinker, the quieter the greener.