In OnStart(), use SetLightVisible(); to make it invisible.
Otherwise in OnStart() and your callback, use FadeLightTo(); instead.
void FadeLightTo(string& asLightName, float afR, float afG, float afB, float afA, float afRadius, float afTime);
Quote:Changes the properties of a light.
asLightName - internal name
afR - red value
afG - green value
afB - blue value
afA - alpha value
afRadius - radius of the light. -1 means keeping the radius
afTime - time in seconds until change is done
The simplest way would be to just copy and paste the R/G/B values you have assigned in your Level Editor (and divide them by 255 to get the decimal float value), but change the alpha to 0 or 1 (invisible and visible), -1 for radii and 0.01f for the time needed.