I have 4 buttons (playback buttons) and 4 pre-selected music-track (song) buttons.
Actually the content is not important.
The screen will always be on like a GPS application.
There will be 2 different set of colors for day and night.
It is easier to find colors for usage while the user is stationary and nothing is moving. However while in car, it both must not attract the eye (so not distract the driver) and also must be visible and quickly separable enough.
I am not a graphic designer and I have no experience and tested-knowledge for best set of colors to use in this situation.
I do not ask directly which colors to use, but I ask what kind of colors to use or is there a site or application that has color-sets for this kind of usage? (somehow like "kuler")