int pSize = 16;
PImage pal = new PImage(pSize,pSize);
color[] colors = new color[pSize*pSize];
void setup()
{
  size(100,100);
  setPal();
}

void draw()
{
  copy(pal,0,0,pal.width,pal.height,0,0,w,h);
}

void setPal()
{
  for (int i = 0; i < 64; i++)
  {
    colors[i] = color(i << 2,255 - ((i << 2)+1),0);
    colors[i+64] = color(255,(i << 2) + 1,0);
    colors[i+128] = color(255 - ((i << 2) + 1),255 - ((i << 2) + 1),0);
    colors[i+192] = color(0,(i << 2) + 1,0);
    pal.pixels[i] = colors[i];
    pal.pixels[i+64] = colors[i+64];
    pal.pixels[i+128] = colors[i+128];
    pal.pixels[i+192] = colors[i+192];
  } 
}