No question is stupid, in my humble opinion. I knew you knew that the scientific info could easily been attained from the internet, so I surmised you were asking for either a poetic or age appropriate description for a young child.or a children's book.
Since you have a good synopis above on the scientific explanation, you could make it even simpler for a little one by stating that the sun works with lifeforces within plants that creates the color that nature intends. Just like the human race all shares some basic traits, genetics combine with the sunshine, good food and exercise to make each of us who we are destined to be. We need the basics to arrive at our full potential, whether we are destined to be tall,or short, fair or dark, Although there are different types of grasses, it becomes the shade of green it is destined to be only with the basics necessary to make it so.
Hope I hit on something that helps, pennyofheaven!,