Google Cardboard: Changing colour of cube based on where I am looking

Hi all,

I have drawn a cube in the scene and I am trying to change the colour of the cube based on where I am looking. Currently I am just trying to measure the distance from the reticle position and the position of the cube. It appears to change the colour once but then never changes it again. Am I using the calculate function incorrectly? Or is there some other facepalm I have missed? Thanks.

import processing.vr.*;
PGraphicsVR p = new PGraphicsVR();
float retX, retY, x, y, z;
color col = color(0, 150, 255);
float d;

void setup() {
  fullScreen(MONO);
  x = width/2;
  y = height/2;
  z = -650;
}

void calculate() {
  //this function is called once per frame immediately before draw
  //use it to update variable values
  retX = p.cameraX;
  retY = p.cameraY;
}

void draw() {
  //this is called twice per frame, once per eye
  background(185);
  lights();

  d = dist(retX, retY, x, y);
  if (d > 150) col = color(255, 0, 255);
  else if (d < 150) col = color(0, 150, 255);
  println(d);

  fill(col);
  translate(x, y, z);
  box(475);

  drawReticule();
}

void drawReticule() {
  eye();
  fill(255, 200, 0);
  stroke(255, 200);
  strokeWeight(5);
  point(retX, retY, 100);
}