Royal Blue, not for it's name just a cool shade plus United won the 68 European Cup playing in that colour of kit.
Interesting fact about that (well it is to me) they were asked to play in a colour other than their traditional red as the contrast on black and white TVs would've been poor against Benfica playing in white.
It's always seemed like a strange question to me because it depends what thing is that colour. I often dress in blue but I wouldn't want to eat a blue biscuit.