The full sentence is "Their small, fine scales are usually brass or bronze in color, sometimes ranging to scarlet, rust, gold, or copper-green." So a red one is kinda like a real life human with ginger hair; uncommon, but not weird. There's also the bit about dragonborn with a particularly strong influence of their draconic ancestor shortly afterwards that says, "These dragonborn often boast scales that more closely match those of their dragon ancestor - bright red, green, blue, or white, lustrous black, or gleaming metallic gold, silver, brass, copper, or bronze."