Why are dragonborn excluded from the “many colors” section of the diagram? They have a wider variety of colors than goblins or tieflings. This is a very poor Venn diagram.

Their small, fine scales are usually brass or bronze in color, sometimes ranging to scarlet, rust, gold, or copper-green. … The blood of a particular type of dragon runs very strong through some dragonborn clans. 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.

- Player’s Handbook, page 32