Draws a picture with 9,999 red squares and one green one

1000.pde

int b = 100;
int s = 640;
int w = b*(s/b);
int h = w;

void setup() {
  background(0);
  size(w, h);
  int side = w/b - 1;
  int x;
  int y;
  noStroke();
  int n = 0;
  for (x = 1; x < w; x += side+1) {
    for (y = 1; y < h; y += side+1) {
      n += 1;
      if (n == b*b/3) {
        fill(0,255,0);
      } else {
        fill(255,0,0);
      }
      rect(x, y, side, side);
    }
  }
}