size(400,400);

color left = color(232,154,28);    // orange
color right = color(155,16,198);   // purple

for (int x=0; x < width; x++) {
  float percent = 1.0 * x / (width-1);         // force floating-point arithmetic
  stroke(lerpColor(left,right,percent));
  line(x, 0, x, height);
}