There's no need to capitalise yankee as it does not mean the same as in the US (and I'm not even sure it's capitalised here?). It means basically juvenille delinquents, and while the word is still used today, it's most strongly associated with the 80s, especially the motorcycle gangs. While dying your hair blond (or brown) has always been something popular with delinquents, there's not really a strong association with dyed hair and yankees (or with heavy makeup, necessarily). The things most associated with yankees (especially in that classic period) would be long school skirts for girls (to the ankle instead of the knee-length most other girls wore) and long school jackets for guys (often worn open with no shirt underneath, just white cloth wrapped around the stomach), regent hairstyles for guys (a type of pompadour I've never seen outside of Japan), and the yankee squat. This guy's got the squat and the hairstyle. One of the girls in the movie Kamikaze Girls is a yankee.

It's not known exactly where the word came from. It seems to have first been used in Osaka and the most common theories are that it might have come from juvenille delinquents at the time in that area liking to wear aloha shirts (thus yankee meaning someone who dresses like someone from the US), or that because the kids used the sentence ending yanke, they were called yanke-ii (people who say yanke), which morphed into yankii (and then took on the Americanised spelling of yankee), or that it was short for young kids (yangu kizzu = yankii).