Fling Ball is a game about flinging balls. Pretty self-explanatory.
As with many Nice Code games, pressing Up, Down, Left, Right, holding A + B (pressing A before B) and pressing Right on the title screen will briefly display a credits screen. Nature Color Game is one of several aliases used by Nice Code, and PJ most likely refers to the Power Joy series of Famiclones which many Nice Code games have been released on.
An alternate version of this game exists which replaces the graphics. The code to access the credits was disabled in this version.