If the Sun put out yellow light instead of white light, rainbows and the spectrum through a prism would not contain all the other colors. Low pressure sodium vapor lamps (the orange-yellow street lights) have mostly yellow/orange output and just a few other spectral emission lines. Ever notice how badly those sodium vapor lights render color? They really suck at it. Try it out for yourself by putting a rainbow or multicolored object under sodium vapor light and observe which colors turn black or gray looking. You will be surprised. The Sun is not and never was yellow.

It's the atmosphere that's changed, not the sun. It's thinner and less protective.

If this were true there would be a mountain of photographic evidence. White balance would change measurably.