Assuming that you draw the ball at some point, you probably have a call to fill() to set the drawing color, and a call to ellipse() to actually draw it. To change the color the ball is drawn in, simply change the fill color again with another call to fill() before you draw the ball with ellipse().

Post your code for more detailed help.

As said above. You can use millis() for the "change later" side, or just frameCount.

Note: please, edit your message to put a better subject line. "Need some help" could be the the subject of some 80-90 % of the messages here... Something like "Problem with bouncing ball" would be better, for example.