Most of the examples aren't even implausible. If the character is a Japanese yanki-type, then it's from a bottle and depicting reality. The same goes for gal and ganguro blondes. Silver is a real hair color, and I've known children with that hair color IRL. Monsters/youkai/Alternate Reality denizens aren't plausible in the first place, so their hair color is up to creator fancy anyway. And finally, some of these examples say that they're "justified", which to me is another word for "plausible".
Fix? Cut examples? Rename "Unlikely Hair Color", or "Uses Hair Dye"? What the heck is this trope?