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
  float r = (1-percent) * red(left) + percent*red(right);
  float g = (1-percent) * green(left) + percent*green(right);
  float b = (1-percent) * blue(left) + percent*blue(right);
  stroke(r,g,b);
  line(x, 0, x, height);
}