I have hooked endscene and I need to draw it here, my end scene is overriding a game call of duty 5 endscene. So I am drawing over it.

How would I go about creating a nice color spectrum such as



Code:
HRESULT APIENTRY my_EndScene( LPDIRECT3DDEVICE9 pDevice )
{
  //DRAW COLOR SPECTRUM HERE
}
I know how to draw it pixel by pixel, but that takes forever and it also slows down your frames per second very very much. So I think there is a built in function to do this for you...