The Sun doesn't actually emit one single "brightest" color. It emits light across the entire visible spectrum, and even beyond, into infrared and ultraviolet.

However, the peak of its emission spectrum is in the green range. This means the Sun emits more green light than any other single color.

But here's the catch: Our eyes aren't equally sensitive to all colors. We're most sensitive to green-yellow, and less sensitive to blue and red. This is why the Sun appears yellow to us, even though it emits more green light.