It is my understanding, though I could be wrong, that you should get a nice blue outline around your beam when you have a properly bench focused unit in sharp focus. This, as has been mentioned, is due to different wavelengths of light refracting at different angles through the
lens. Here is an image from Wikipedia:
You can see that the colors with shorter wavelengths (the blue and UV end of the spectrum) converge at a focal point closer to the lens, thus, ending up at the outside of the field when you focus a light sharp. Since the reds end up inside the bulk of the field we don't tend to see them. However, as you move the lens to focus and you get closer to the different focal lengths of the different wavelengths you may see some of that color show up on the edge of the field. Hence the red-brown "halo."