Processing 3D 図形を重ねた時に色が混ざる

processingで3Dの図形を色を混ぜずに重ねたいです．

実行直後の向き（正面から）だと1枚目の画像のようになるのですが，マウスを動かして角度を変えると2枚目の画像のように黄色と黒が混ざってしまいます．

常に黒が上に来るようにしたいのですが，どうすればよいでしょうか？

/* @pjs font="TEMPSITC.TTF"; 
 */

boolean isGrid = true, isAxis = true;
void setup(){
  size(500, 300, P3D);
  noStroke();
  textFont(createFont("Tempus Sans ITC", 24));
  frameRate(10);
}

void axis(char s, color c){
  int len = 200;
  fill(c);
  stroke(c);
  if(isAxis){
    box(len, 1, 1);
    pushMatrix();
      translate(len / 2, 0, 0);
      sphere(3);
      text(s, 5, -5, 0);
    popMatrix();
  }
  if(isGrid){
    pushMatrix();
      translate(0, -len / 2, -len / 2);
      int ngrids = 20, xs = len / ngrids, ys = len / ngrids; 
      strokeWeight(1);
      for(int i = 1; i < ngrids; i++){
        line(0, 0, ys * i, 0, len, ys * i); 
        line(0, xs * i, 0, 0, xs * i, len); 
      }
    popMatrix();
  }
}
void drawAxis(char s, color c){
  switch(s){
    case 'X':
      axis(s, c);
      break;
    case 'Y':
      pushMatrix();
        rotateZ(PI / 2);
        axis(s, c);
      popMatrix();
      break;
    case 'Z':
      pushMatrix();
        rotateY(-PI / 2);
        axis(s, c);
      popMatrix();
      break;
  }
}

void coloredCube(){
  pushMatrix();
    scale(.5, .5, .5);
    noStroke();
    beginShape(QUADS);
    fill(245, 223, 77);
    vertex(1, -1, -1); vertex(1, -1, 1); vertex(1, 1, 1); vertex(1, 1, -1);
    vertex(1, 1, 1); vertex(-1, 1, 1); vertex(-1, 1, -1); vertex(1, 1, -1);
    vertex(-1, 1, 1); vertex(1, 1, 1); vertex(1, -1, 1); vertex(-1, -1, 1);
    vertex(-1, -1, -1); vertex(-1, -1, 1); vertex( 1, -1, 1); vertex( 1, -1, -1);
    vertex(-1, 1, -1); vertex(-1, 1, 1); vertex(-1, -1, 1); vertex(-1, -1, -1);
    vertex(1, 1, -1); vertex(-1, 1, -1); vertex(-1, -1, -1); vertex(1, -1, -1);
    endShape();
  popMatrix();
}

void pillar(float length, float radius1 , float radius2, int r, int g, int b){
float x,y,z;
pushMatrix();
  noStroke();
  fill(r,g,b);
  //top
  beginShape(TRIANGLE_FAN);
    y = -length / 2;
    vertex(0, y, 0);
    for(int deg = 0; deg <= 360; deg = deg + 10){
    x = cos(radians(deg)) * radius1;
    z = sin(radians(deg)) * radius1;
    vertex(x, y, z);
  }
  endShape();              
  //bottom
  beginShape(TRIANGLE_FAN);
    y = length / 2;
    vertex(0, y, 0);
    for(int deg = 0; deg <= 360; deg = deg + 10){
      x = cos(radians(deg)) * radius2;
      z = sin(radians(deg)) * radius2;
      fill(r,g,b);
      vertex(x, y, z);
  }
  endShape();
  //side
  beginShape(TRIANGLE_STRIP);
    for(int deg =0; deg <= 360; deg = deg + 5){
      x = cos(radians(deg)) * radius1;
      y = -length / 2;
      z = sin(radians(deg)) * radius1;
      vertex(x, y, z);
      x = cos(radians(deg)) * radius2;
      y = length / 2;
      z = sin(radians(deg)) * radius2;
      vertex(x, y, z);
    }
  endShape();
popMatrix();
}
void drawShape(){
  pushMatrix();
    translate(0, 20, 0);
    scale(100, 40, 60);
    coloredCube();
  popMatrix();
  pushMatrix();
    translate(40, 0, 0);
    rotateX(HALF_PI);
    fill(0);
    pillar(60,20,20,0,0,0); 
  popMatrix();
  pushMatrix();
    translate(-40, 0, 0);
    rotateX(HALF_PI);
    pillar(60,20,20,0,0,0); 
  popMatrix();
}
void draw(){
  background(255);
  translate(250, 150, -50);
  rotateX(map(mouseY, 0, height, PI, -PI));
  rotateY(map(mouseX, 0, width, -PI, PI));
  pushMatrix();
    drawAxis('X', color(255, 0, 0, 60)); 
    drawAxis('Y', color(0, 255, 0, 60)); 
    drawAxis('Z', color(0, 0, 255, 60)); 
    drawShape();
  popMatrix();
}