// This implementation makes use of a boolean variable 'touching' that
// is changed whenever the balls first create contact or subsequently separate

int radius = 20;
color c = color(255);
boolean touching = false;

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

void draw() {
  background(204);
  if (dist(width/2, height/2, mouseX, mouseY) <= 2 * radius) {
    if (!touching) {
      touching = true;
      c = color(random(255), random(255), random(255));
    }
  } else {
    touching = false;
  }
  fill(0);
  ellipse(width/2, height/2, 2*radius, 2*radius);
  fill(c);
  ellipse(mouseX, mouseY, 2*radius, 2*radius);
}