The problem with hborre's setup is that an ambient value of >1 will change the color of the emitter prop. In this case, the orb looks yellow while it casts an orange glow.

So for my render I used a second orb, slightly bigger than the first one.

The first one has an ambient value of 5, but is made invisible to the camera, so only its "glow" is visible in the final render.

The second one has an ambient value of 1 and is not visible in raytrace, but in camera.

So it just renders normally, but doesn't block the glow of the first orb hidden inside it.

This way, the orb glows red while it still "looks" red at the same time.