Matching shoes with colors

How can I know when to wear black shoes or brown shoes so that it matches with what I am wearing? I know how to use the color wheel to match colors. But since the color wheel doesn’t have black and brown on it, I’m not sure what shoes to wear with certain colors. For example, with a green shirt, I’m pretty sure brown shoes would work, but would black also be acceptable?

How Can I Dress Down A Contrast Collar Shirt?

I just recently got an internship position at the district office of a local politician. I was told their dress policy was slightly relaxed and included dress shirts and dress pants, but ties and suits aren’t needed.

I also recently bought a blue dress shirt with a white contrasting collar and contrasting French cuffs. My question is, would this shirt be too formal for an intern when the the dress code is business casual? Would something like leaving the top two buttons unbuttoned and rolling up the sleeves using the basic or master roll be acceptable, or is it best to just not try dressing down this style of shirt and leave it for a more formal setting?

Thank you.