// This implementation looks for a time when the current ball location
// is contacting the fixed ball, but based on the previous mouse location it was not.

int radius = 20;
color c = color(255);

void setup() {
  size(500,400);
}

void draw() {
  background(204);
  if ((dist(width/2, height/2, mouseX, mouseY) <= 2 * radius) &&
      (dist(width/2, height/2, pmouseX, pmouseY) > 2 * radius)) {
    c = color(random(255), random(255), random(255));
  }
  fill(0);
  ellipse(width/2, height/2, 2*radius, 2*radius);
  fill(c);
  ellipse(mouseX, mouseY, 2*radius, 2*radius);
}