I tried to posterize a picture but when i clicked the mouse for the effect to happen. the error appears: arrayindexoutofboundsexception

the program is like this:
int i;
float total, r, g, b;
PImage img;

void setup(){
  size(427,640);
  background(255);
 img=loadImage("Hey.jpg");
 image(img, 0, 0);
 
}
void draw(){
   
}
void mouseClicked(){
  img.loadPixels();
  for(i=0; i<img.pixels.length; i++){
    r=red(img.pixels[i]);
    g=green(img.pixels[i]);
    b=blue(img.pixels[i]);}
   total=r+g+b;
  if (total<182)
  img.pixels[i]=color(0, 51, 76);
  else if (total<364)
  img.pixels[i]=color(217, 26, 33);
   else if (total<546)
  img.pixels[i]=color(112,150,158);
   else
  img.pixels[i]=color(252, 227, 166);
 
  img.updatePixels();}