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This for the classic LR rule set:

EDIT: I made a version which renders 4096X4096 images, using 2^24 colors. The colors are also 'reflected' so they make nice, smooth gradients. I'll only provide links to them since they're huge (>28 MB)

Langton's Rainbow large, rule LR

Langton's Rainbow large, rule LLRR

//End of edit.

This for the classic LR rule set:

#include "CImg.h"
using namespace cimg_library;
CImgDisplay screen;
CImg<unsigned char> surf;
#define WIDTH 256
#define HEIGHT 128
char board[WIDTH][HEIGHT];

class ant
{
  public:
  int x,y;
  char d;
  unsigned short color;
  unsigned int unvisitted;
  void init(int X, int Y,char D)
  {
    x=X;y=Y;d=D;
    color=0;
    unvisitted=WIDTH*HEIGHT;
  }

  void turn()
  {
    //LRRRRRLLR
    char get=board[x][y];
    if(get==0||get==6||get==7){d+=1;}
    else{d-=1;}
    if(d<0){d=3;}
    else if(d>3){d=0;}
  }

  void forward()
  {
    if(d==0){x++;}
    else if(d==1){y--;}
    else if(d==2){x--;}
    else {y++;}
    if(x<0){x=WIDTH-1;}
    else if(x>=WIDTH){x=0;}
    if(y<0){y=HEIGHT-1;}
    else if(y>=HEIGHT){y=0;}
  }

  void draw()
  {
    if(board[x][y]==-1)
    {
      board[x][y]=0;
      unsigned char c[3];
      c[0]=255-((color&0x1F)*8);
      c[2]=255-(((color>>5)&0x1F)*8);
      c[1]=(((color>>10)&0x1F)*8);
      surf.draw_point(x,y,c);
      unvisitted--;
      color++;
    }

    board[x][y]++;
    if(board[x][y]==9){board[x][y]=0;}

  }

  void step()
  {
    draw();
    turn();
    forward();
    //screen=surf;
    //screen.wait(1);
  }
};

void renderboard()
{
  unsigned char white[]={200,190,180};
  surf.draw_rectangle(0,0,WIDTH,HEIGHT,white);
  for(int x=0;x<WIDTH;x++)
  for(int y=0;y<HEIGHT;y++)
  {
    char get=board[x][y];
    if(get==1){get=1;unsigned char c[]={255*get,255*get,255*get};
    surf.draw_point(x,y,c);}
    else if(get==0){get=0;unsigned char c[]={255*get,255*get,255*get};
    surf.draw_point(x,y,c);}
  }
}

int main(int argc, char** argv)
{

  screen.assign(WIDTH*4,HEIGHT*4);
  surf.assign(WIDTH,HEIGHT,1,3);
  ant a;
  a.init(WIDTH/2,HEIGHT/2,2);
  surf.fill(0);
  for(int x=0;x<WIDTH;x++)
  for(int y=0;y<HEIGHT;y++)
  {
    board[x][y]=-1;
  }

  while(a.unvisitted>0)
  {
    a.step();
    //renderboard();
  }

  screen=surf;
  while(screen.is_closed()==false)
  {
    screen.wait();
  }
  surf.save_bmp("LangtonsRainbow.bmp");
  renderboard();
  surf.save_bmp("LangtonsBoard.bmp");
  return 0;
}

#include "CImg.h"
using namespace cimg_library;
CImgDisplay screen;
CImg<unsigned char> surf;
#define WIDTH 256
#define HEIGHT 128
#define TOTAL WIDTH*HEIGHT
char board[WIDTH][HEIGHT]; 


class ant
{
  public:
  int x,y;
  char d;
  unsigned short color;
  void init(int X, int Y,char D)
  {
    x=X;y=Y;d=D;
    color=0;
  }

  void turn()
  {
    ///Have to hard code for the rule set here.
    ///Make sure to set RULECOUNT to the number of rules!
    #define RULECOUNT 9
    //LRRRRRLLR
    char get=board[x][y];
    if(get==0||get==6||get==7){d+=1;}
    else{d-=1;}
    if(d<0){d=3;}
    else if(d>3){d=0;}
  }

  void forward()
  {
    if(d==0){x++;}
    else if(d==1){y--;}
    else if(d==2){x--;}
    else {y++;}
    if(x<0){x=WIDTH-1;}
    else if(x>=WIDTH){x=0;}
    if(y<0){y=HEIGHT-1;}
    else if(y>=HEIGHT){y=0;}
  }

  void draw()
  {
    if(board[x][y]==-1)
    {
      board[x][y]=0;
      unsigned char c[3];
      c[0]=255-((color&0x1F)*8);
      c[2]=255-(((color>>5)&0x1F)*8);
      c[1]=(((color>>10)&0x1F)*8);
      surf.draw_point(x,y,c);
      color++;
    }

    board[x][y]++;
    if(board[x][y]==RULECOUNT){board[x][y]=0;}

  }

  void step()
  {
    draw();
    turn();
    forward();
  }
};

void renderboard()
{
  unsigned char white[]={200,190,180};
  surf.draw_rectangle(0,0,WIDTH,HEIGHT,white);
  for(int x=0;x<WIDTH;x++)
  for(int y=0;y<HEIGHT;y++)
  {
    char get=board[x][y];
    if(get==1){get=1;unsigned char c[]={255*get,255*get,255*get};
    surf.draw_point(x,y,c);}
    else if(get==0){get=0;unsigned char c[]={255*get,255*get,255*get};
    surf.draw_point(x,y,c);}
  }
}

int main(int argc, char** argv)
{

  screen.assign(WIDTH*3,HEIGHT*3);
  surf.assign(WIDTH,HEIGHT,1,3);
  ant a;
  a.init(WIDTH/2,HEIGHT/2,2);
  surf.fill(0);
  for(int x=0;x<WIDTH;x++)
  for(int y=0;y<HEIGHT;y++)
  {
    board[x][y]=-1;
  }

  while(a.color<TOTAL)
  {
    a.step();
  }

  screen=surf;
  while(screen.is_closed()==false)
  {
    screen.wait();
  }
  surf.save_bmp("LangtonsRainbow.bmp");
  return 0;
}