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."
Rare, yeah, but still a valid possibility. But the main part I wanted to bring up is that the sentence you were referring to actually already includes "scarlet" as an ordinary colour, so the red one in the accompanying picture fits just fine
Or a one note one. But, while I like monstrous races as options, I dislike the trend of 5e to make our characters "special", unique, or noteworthy before the adventuring even begins.
(If this is duplicated for some reason, I'm sorry. It tried editing and that didnt' seem to take, then I tried deleting my original message and reposting. Not sure what's up.)