int i;
size(300,300);

for(i=0; i<300; i+=3){

 stroke(255,0,i);
 line(i,0,i,300);
 line(i,0,300,i);

}

for(i=0; i<300; i+=3){

 stroke(0,i,255);
 line(0,0,i,300);

 stroke(i,255,0);
 line(i,0,300,i);

}