Now, I can already hear heads being scratched out there. Not at all. Let s say you have a scene with nothing but a standard shape plane in it and the default light. The plane will appear to be shades of gray, as it is a white object colored by a white light. Now, if you change the color of the light to be yellow, your plane will appear as shades of yellow.
Light color is set to determine the color of the light being cast onto other objects.
Now, look at the snapshot render you just did. Not very much. Only the white highlights from the sides of the letters appear white, as they are from a white object with white light cast on it. The green is coming from the glow entirely. We are simply looking down the cone of a light and looking at its glow.