// int col          - Variablen & Datentypen
// col = col - 1    - Zuweisung, Änderung
// if, else         - Bedingungen

int col = 255;

void setup() {
  size(200, 200);
  smooth();
}

void draw() {
  background(124);
  if (mousePressed) {
    col = col - 1;        // oder: col--;
  }
  else {
    col = 255;
  }

  fill(255, col, col);
  ellipse(mouseX, mouseY, 60, 60);
  fill(255);
  ellipse(mouseX - 10, mouseY - 10, 20, 20);
  ellipse(mouseX + 10, mouseY - 10, 20, 20);
}