boolean finish = false;

void setup() {
 size(400,400);
 colorMode(HSB,360,100,100,100);
}

void draw() {
 if( !finish ) { 
   color cFrom = color(0,0,100);
   color cTo   = color(int(random(360)),100,100);
   drawVirtGrad(cFrom, cTo);
   finish = true;
 }
}

void drawVirtGrad(color from, color to) {
 float step = 1.0 / height;
 for(int y = 0 ; y < height ; y++ ) {
   color c = lerpColor(from, to, step * y);
   stroke(c);
   line(0,y,width,y);
 }
}