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."
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."
You omitted that the draconic colored scales are from extremely rare and isolated clans.
Which is exactly the sort of origin that makes a fun character.