I'm doing this from memory as I'm getting ready to walk out the door and it's been a little while since I've done this effect, so if it isn't quite right, I'll take another stab at it when I get home from work. I think it's something along these lines, though:
iLuxEntity @lamp = Map_GetEntity("lamp"); // The name of the entity in the level editor
iLight @light = lamp.GetLightFromName("light"); // The name of the attached light in the model editor
light.SetIlluminationColor(cColor(0.0, 0.0, 1.0)); // RGB values, 0-1 scale