int x,y,xc,yc;
void setup(){
  size(255,255);
  noStroke();
}
void draw(){
  x=mouseX;
  y=mouseY;
  xc=255-x;
  yc=255-y;
  fill(x,y,0);
  rect(0,0,255,255);
  fill(xc,yc,255);
  rect(50,50,155,155);  
}