I've been there multiple times and can confirm. Among the younger population it's like 90% black and white clothing. That number goes down as you look at older groups. However, almost all of the cars are also black or white.

I stuck out like a sore thumb wearing pink, orange, green, etc. Last time I went, I was in Seoul for a whole week. I saw one young person wearing pink, and maybe 10 young people wearing colorful outfits.