white onion versus yellow onion
White and yellow onions share a similar shape, but that's as far as it goes. White onions have a sharp flavor that is clean and sweeter than yellow onion. White onions are more moist than yellow onions, which makes them firmer than their yellow cousin. They are good in their raw form but they actually improve in flavor when included as a cooked ingredient; hence they work very well in Mexican dishes and they are mild enough in raw form to make them perfect for salsa, etc.
Yellow onions are stronger (more pungent) than white onions and they have a very high sulphur content which makes them more likely to produce teary eyes when preparing as an ingredient in cooked foods. Because they are strong they are less popular in their raw form (although my preference for strong onion flavor means I actually prefer them raw) but this is an advantage when you want a stronger onion flavor in recipes that seek to emphasize onion flavor. They caramelize beautifully (better than white onions).