What is the result of combining all colors of the visible spectrum?
The simple answer is "white" light, but it depends...
One of my favourite questions to perplex those with a passing aquaintance with physics is "Why does red light plus green light give you yellow light?"
The thing is that pure yellow light has a frequency somewhere between red and green light. So how can longer and shorter waves somehow combine to give you something in between?
The effect upon our eyes of a combination of pure red and pure green light is similar to the effect of pure yellow light.
Regarding the current question: If all of the colours of the visible spectrum are combined in appropriate proportions, the resulting colour perceived will be white.
The universe is officially beige.
"All the colours of the rainbow" does not include magenta.