Many serial costumes were actually done in shades of grey; and, I'm fairly certain these were. The only color would have been painted illustrations seen on movie posters and lobby cards. The George Reeves Superman costume was greay, as was Tom Tyler's Captain Marvel costume in the Adv. of Captain Marvel serial.
"Fortunately, Ah keep mah feathers numbered for just such an emergency!"

I don't know if any color photographs were taken of them in color. Costumes and props in black and white movies were chosen based on how they would look in black and whiteso the colors didn't necessarily reflect how they appeared in the comics. It's an interesting question. I'd love to see what the costumes looked like in color.