Cooking question: What method could I use to saute onions without turning the final dish a nasty grey color?

I used onions sauteed in butter for my onion soup. That soup is just heavenly, but it looks gross because it’s all grey.
This past Christmas I tried baking a home made stuffing. I sauteed the onions along with mushrooms and other stuff in butter, the way I do when I’m putting it in the bird, and the stuffing turned a nasty grey color. It tasted really good though, but not many people tried it.